A Mountain View teacher generated national headlines last week after it appeared he was punished by his school district for comparing the campaigns of Donald Trump and Adolph Hitler.

Now Frank Navarro is back at work and demanding an apology.

Officials from the Mountain View-Los Altos Union High School District said Wednesday the history teacher was indeed put on administrative leave last week. But they contend it had nothing to do with the controversial lesson.

Navarro, who has taught at Mountain View High School for about 40 years, was abruptly sent home last week on paid administrative leave. He claims it stemmed from the Trump-Hitler lesson, which he believes was misinterpreted by a parent who then filed a complaint with the district.

The district, however, says Navarro was sent home for what they called a separate confidential personnel matter. District Superintendent Jeff Harding said there was a thorough confidential investigation into the separate issue that Navarro is fully aware of.

That has Navarro upset.

"The explanation has been that they were concerned for the health of the students, and I have not seen any indication that the students have been imperiled," Navarro said.

Navarro said he's mystified and that the ordeal has taken a toll. He has vowed to quit at the end of the school year.

"This will be my last year," he said. "I am going to retire; this really did push me out the door."

An Change.org online petition in support of Navarro and addressed directly to Mountain View High Principal David Grissom has generated nearly 40,000 signatures. Despite Navarro's plan to resign, supporters say they have no intention of halting the campaign.

District officials had no comment on the petition and said they consider the case closed.