(CNN) Two dozen students marched out of their classroom at Drexel University in Philadelphia last month carrying signs that read "Where is our professor?" and "Bring back GCM."

They felt they had a pressing reason to protest.

"For 40-plus days, we've been without a professor in the classroom," said Dakota Peterson, a 22-year-old political science student.

Their professor, George Ciccariello-Maher, has been teaching his class remotely via video conference after the university put him on administrative leave. He's not allowed on campus without a police escort. The school says his enforced absence is due to safety concerns raised by a furious and frightening reaction to controversial posts he made on social media.

Ciccariello-Maher, who teaches politics and global studies, said he's received numerous death threats over the past 12 months.

"I have 800 unread voicemails in my inbox right now that have been building up over the past few weeks. And this ... it's just something that happens all the time," he said.

"Threats that involve my child are, of course, the ones that are the most frightening to me."

'White Genocide' tweet sparks fury

Ciccariello-Maher says the threats began last December after he posted on Twitter: "All I Want for Christmas is White Genocide." He says the tweet was a joke -- a "satirical jab at a certain paranoid racist fantasy and that white genocide does not exist."

In March he tweeted that he was "trying not to vomit" watching someone give up their first class seat for a uniformed soldier. He says he was upset about airstrikes in Iraq that reportedly killed at least 200 civilians in Mosul as part of an effort to drive out the Islamic State, telling the Daily News that his post was "misrepresented by the outrage machine that is right-wing media," and that his views were expressed off campus.

And, in October, he tweeted that "Trumpism" and the "narrative of white victimization" were to blame for the Las Vegas mass shooting that claimed 58 lives.

His tweets have gone viral and outraged many, sparking a bitter debate that raises a question for schools nationally: Should a professor's comments, however inflammatory, be treated as free speech and protected under the banner of academic freedom

The American Association of University Professors told CNN in an email that "academic freedom" should include "the freedom of faculty members to speak on matters of public concern." At public universities, the First Amendment may protect faculty from disciplinary action for "off-duty" speech on matters of public concern, they said. But the group acknowledges that at private institutions, the First Amendment provides no such protection.

Conflict over free speech on college campuses has intensified visibly during the last year. Violence has broken out surrounding protests staged when controversial speakers, including far-right provocateur Milo Yiannopoulos and white nationalist Richard Spencer, have spoken -- or announced plans to speak -- at universities. There's a reason groups on both sides of the political spectrum are so focused on what college students are hearing.

"This is where the next generation's perspectives and ideas will be decided," says 24-year-old Charlie Kirk, founder of Turning Point USA, an advocacy group for young conservatives. "It's happening on college campuses right now. That's why you're seeing the protests. That's why you've seen the riots, where you see record turnout for conservative speakers that come on campus because there's so much interest out there."

Comments weaponized

In the past year, more than 100 incidents of targeted harassment against professors have been reported on college campuses, according to the American Association of University Professors.

-- A California professor at Orange Coast College who was captured on video in the classroom criticizing President Donald Trump got death threats, according to The Washington Post

-- CBS News reported a Princeton professor received death threats after giving a commencement speech in which she called Trump a "racist and sexist megalomaniac."

-- A University of Iowa professor was targeted after discussing white supremacy in the context of ancient statues and their use of white marble, InsideHigherEd.com says.

"Historically, it's been conservative professors who have faced retaliation and disciplinary threats, and harassment for offending people," said Ari Cohn, director of the Individual Rights Defense Program for the nonprofit Foundation for Individual Rights in Education.

"I think over the course of the past year, as liberal faculty members have become increasingly outspoken with their criticism of the Trump administration, you have seen more professors on the left being targeted."

Groups targeting professors

Professors who thought that their classrooms and social media forums are safe places to discuss controversial ideas are learning they have a much wider and more vindictive audience than they imagined.

"There's an entire cottage industry for reporting on controversial things that faculty members say, which then riles up Internet outrage mobs, who then start to tweet at schools, or post on Facebook, and try to get schools to get rid of people who they disagree with," Cohn says.

One of the websites, called Professor Watchlist, was started by Charlie Kirk.

"Our goal is to expose and really profile professors that have engaged in outward radical behavior," said Kirk. "We're not saying these professors should be silenced. Instead, we're pointing out what has become a systemic problem in our universities, where professors are able to say things that are completely outrageous."

Kirk said that every few weeks new professors are added to the online list. Other sites like Campus Reform and College Fix have similar goals, with articles on one site often appearing on the other.

When it comes to the harassment and death threats that professors face, Kirk says it's not his website but the professors themselves who bear responsibility for the consequences of their words.

"We do not call for any that sort of harassment. We don't condone it, we don't try to facilitate any sort of cyberbullying or harassment, and just because you put up the words, or another article that's been written about a professor in an aggregated format, does not mean we should be held responsible for what other people do."

Harassment laws 'mostly ineffective'

Exactly what protection against targeted harassment the law can provide is unclear, Miami School of Law professor Mary Anne Franks told CNN.

"Professors have essentially the same legal options as any person targeted by online harassment -- which is to say, few and mostly ineffective. While threats and harassment are technically prohibited by both federal and state laws, and there are a variety of civil laws that could also apply. ... They are rarely used except in the most extreme cases, and often only after some physical harm has already occurred," Franks told CNN in a statement.

In general, law enforcement does not take complaints of online harassment and threats seriously, Franks said. Police departments often lack the resources and technological competence to investigate, she said, and many departments believe online speech doesn't lead to real-world action.

But Franks thinks the increasingly toxic environment will eventually demand closer scrutiny from the authorities.

"Given the nature of online anonymity and the sheer volume of threats and abuse, it is clear that law enforcement must not only start taking threats and harassment more seriously, but that we also need to implement large-scale solutions against the sites and social media users who incite online mobs," she said.

An interesting argument

"I hope you get what's coming to you ... especially if you're a n*****. Go f*** yourself."

That's just one of the many hateful voicemails professor Johnny Williams, who teaches sociology at Trinity College in Hartford, Connecticut, has received this year.

This summer Williams shared an article about letting racial bigots die, using the hashtag #LetThemF***ingDie. Websites including Campus Reform and Professor Watchlist publicized his comments.

Williams says his post was misinterpreted and that his "only aim was to bring awareness to white supremacy."

"I was only quoting an article by a guy that had it entitled, 'Let Them F'ing Die.' ... He wasn't talking about all white people. He was saying, 'What if racial bigots who don't care anything about us and let us die -- what if we ignore them and let them die?' That was his particular concern. I thought it was an interesting argument," said Williams.

The threats haven't stopped since June, he said.

"I get death threats ... they called me a coward. They called for my wife to be raped, and my kids to be shot in the head. The death threats and the harassments continue, it hasn't stopped.

"This one guy, he called in (and) all he did was say, 'N*****, n*****, n*****, n*****, n*****, n*****, n*****, n*****, n*****, n*****,' just kept saying that for about a minute then he hung up. Others call and say, 'I know where you are.'"

Schools struggle with response

Because of the threats, the police got involved and Trinity College shut down its campus for a half-day in June. Williams was placed on administrative leave.

"We were very concerned, about the death threats that we were receiving on campus," said school President Joanne Berger-Sweeney. "We decided that we needed time to assess the facts of the case."

Williams, she said, will be back on campus to teach next semester.

"I think the intent of many of these websites is to create internal disturbance within higher education," Berger-Sweeney said. "They probably would like nothing more than for professors to be attacking administration, administration to be concerned with what faculty members are doing, students to be attacking the administration. Because if you want a system to decay, you want it to decay from the inside. That is very effective."

Williams said he felt betrayed by his college's response and believes universities need to do more to protect the speech of academics like him.

"I think the first thing they need to do is say, 'Our professors are here to profess, to provoke these thoughts to advance knowledge and this is a part of it.'

"I'm learning from the people out there and I'm learning from people here at the college as well, so it's a place just to learn."

The marketplace of ideas

Not everyone thinks professors should be able to say whatever they like.

"Having a personal opinion is fine, but when you come out and you make these comments, you're a representation of the university, and that goes for anything. If you're on a sports team, you're representing the sports team, and you're held accountable for your actions. If you are a teacher, you're held accountable for your actions," said Nick Nanakos, a business student who has been troubled by what Drexel's has had to say.

But Ciccariello-Maher stands by his controversial tweets and says he plans to continue to speak out.

"I mean these are political debates, right? These are things that we need to be talking about ... the idea that middle-class white men are victims when they're not, when they're very much overrepresented and privileged in American society. These are conversations we need to have," he said.

In a statement to CNN last week, the school said the professor had been placed on administrative leave and that arrangements had been made for him to teach his courses online. It emphasized its commitment to academic freedom while ensuring a safe campus environment.

But Thursday, Ciccariello-Maher said he was resigning because the harrassment and threats had become too much.

"After nearly a year of harassment by right-wing, white supremacist media outlets and Internet mobs, after death threats and threats of violence directed against me and my family, my situation has become unsustainable," he wrote on his Facebook account. "Staying at Drexel in the eye of this storm has become detrimental to my own writing, speaking, and organizing."

The university said Thursday it wishes Ciccariello-Maher well.

"Drexel University has accepted his resignation and recognizes the significant scholarly contributions that Professor Ciccariello-Maher has made to the field of political thought and his service to the Drexel University community as an outstanding classroom teacher," the school statement said.

The risk of backlash online, death threats, and uncertainty about how universities will respond to issues involving free speech in education has many academics concerned.

"The real danger is that they decide that it's no longer worth it to engage in the marketplace of ideas, and to be those public scholars that they're supposed to be, and instead simply remain quiet for fear that what they say might get them fired," said Cohn.