“The truth is, us kids, we just want to be the voice for the people we lost, or for people who don’t think they have a voice,” said Ms. Fuentes, who would like to eventually become an elementary schoolteacher. “I am prioritizing this over college right now, I am prioritizing people’s lives over my education.”

University admissions offices declined to discuss individual applications, but many noted that civic engagement is a plus in considering whether to admit an applicant. In fact, some colleges publicly assured high school applicants in advance of last weekend’s March for Our Lives rallies that their participation in peaceful protests would not harm admission chances.

Harvard, for example, issued a statement: “The mission of Harvard College is to provide a deeply transformative liberal arts and sciences education that will prepare our students for a life of citizenship and leadership. Fundamental to our mission is our belief that students have the right to protest peacefully about issues of concern to them. Students who are disciplined for engaging responsibly in exercising their rights and freedoms would not have their chances of admission compromised or their admissions revoked.”

While on spring break this week, Mr. Hogg said he was leaning toward taking a gap year to focus on the midterm elections, hoping to rally young voters and target politicians supported by the National Rifle Association.

“We are trying to get these laws changed to drive down the number of people who die by gun violence,” he said. “I can figure out everything else later.”

Mr. Hogg said he does not believe his outspokenness had anything to do with the college denials, but admits it has been frustrating. In a late-night, March 16 message on Twitter — liked by at least 64,000 and retweeted nearly 6,600 times — Mr. Hogg told his followers, “Just got rejected from another college but that’s ok we’re already changing the world. Goodnight everyone.”

He added that he was comforting himself by eating chocolate muffins. The response was mixed, with some mocking his rejection, some encouraging him to take a gap year and others touting him as precisely the type of student colleges should recruit.