The lawsuit states that Krolikowski was unlawfully discriminated against because he did not conform to the school's perception of how a male should dress and act.
"His appearance hasn't changed very much, so no one understands why this happened," Guarino said. "It wasn't an issue for 32 years. Why is it a problem now? It seemed very sudden."
The termination, the lawsuit says, violates New York state and city law.
New York state's human rights law states that an employer cannot discharge an employee because of "an individual's age, race, creed, color, national origin, sexual orientation, military status, sex, disability, predisposing genetic characteristics, marital status, or domestic violence victim status."
Carmelyn Malalis, an LGBT workplace lawyer with Outten & Golden LLP in New York, said the law has been used to protect transgender people.
New York City's Human Rights Law prohibits discrimination in hiring and firing based on similar attributes but specifically mentions gender identity.
Krolikowski's story has made headlines. And the online petition has grown to about 3,000 signatures.
One post from David Burkard in Queens Village, New York, said this:
"We the students/alumni/ and staff of Prep are supposed to be good 'Christians' and that means not only going the extra step to be kind to others, but also that we must be accepting and loving to EVERYONE. Mr. K always spoke out about public injustices, both domestic and international, and by doing so proved to be one of the MOST 'Christian' persons I've ever met.
"You cannot just fire someone because they 'don't reflect the values you wish to instill,' when clearly they do. The act of firing Mr.K has proved to me that the establishment is quite plainly ignorant, insensitive, and most importantly hypocritical. I was once a proud Terrier, but now I can honestly say I'm ashamed."