while i understand what you're asking for Merlin, i don't really know of one, you might want to think of something else. plus, i don't see how this would work, there are guys who dream about the same sex, same as some girls do, and they're straight. there's plenty of reasons why this would occur, but people, guys especially, would never talk about it, so i don't see how ashamed and guilty they'd feel. in fact, they might lash out to prove themselves not gay in their mind if they're so shallow as to insult someone by a glance.
you could try a protection spell on your friend, a stop gossip spell, a honey jar spell perhaps to make them sweeten up. if i were you i'd ignore them. i had to deal with similar people in high school. i'm not gay, but i was a member of my schools gay straight alliance along with some friend, and we got a lot of crap as a result of having fundraisers to support AIDS research, putting up posters of famous gay people, having movie nights showing films like Philadelphia and Rent. one of the teachers who ran the GSA got the school board to green light The Laramie Project. got a lot of heat from that one, even Fred Phelps wanted to come to my town to protest the play. point is, i understand close minded people like that, making them think they have homosexual tendency probably won't make them avoid you.
i'd do a protection and a stop gossip spell if you don't want to hex them. non-magically, i'd ignore them, or try to document what's going on so if it escalates you have evidents to present to the teachers, principal, or school board depending on how bad it gets.