Basically, "we have a section to write about any experiences we have had with homosexuality" is a pretty big deal; that's about as private as private gets, along with how many bases you've gotten to, if any, with her or any other human being.
I don't think that's anybody's business but yours and your parents, should it come to their attention. It strikes me as pretty weird that a first-year teacher thinks that this is an appropriate assignment and frankly I'll bet her superiors don't know and that your parents wouldn't be happy to hear about it.
I'm with Cosmitz on this; make stuff up.
Your sex life is not their business, unless you voluntarily bring it up in Confession (provided you're Catholic).
So tell them some lame story about a 1-hour summer romance with a cute gal in a truckstop in Idaho (see, pretty easy) who was the first woman to ever ask you for your number because you bought that really silly Cows R Kool t-shirt and your little brother was singing that annoying-but-cute number from (insert Anime here) and you bought him a soda at the trucker's counter, which she thought was really sweet.
In fact, feel free to plagiarize all that and embellish it, and voila, problem's solved
