I had a similar experience.
At lunch this group of girls that I usually sit and eat with, earlier I had explained to one of them that I'm FTM. The first one started whispering it to another one, and it kept going. They were laughing at me and making fun of me, acting like this was all just a game to them. They asked me if I was kidding, and I tried to explain to them the best I could, but they just ended up excluding me.
It hurt me terribly, but I'm glad I'm past that.
My school is largely uneducated on transgendered people. I try and explain it to them, but people usually just end up making fun of me or excluding me.