[Retroman]

Are we talking school kids here?
It seems that many people of a certain age aren't trustworthy or realise the consequences of gossiping. They'll have their assumptions and thoughts about bisexuals firmly built into their brains, and most of them won't start to accept and understand different people until they're older.

So in my honest opinion, very personal details such as your sexuality have no place in being kept secret amongst school students. As most aren't capable of keeping their mouths shut, and only have an understanding of talking about people, not keeping things to themselves. It's just my advice, but I wouldn't trust any "friends" with anything until at least after school, when everyone's forced to do a little growing up.

As for the friends, it seems even the good friend is more than willing to talk about you behind your back? Also, I can only imagine how it was discussed.
Eg. "Oh yea guess what, he's bisexual!" "No! really? haha" *they proceed to discuss your sexuality and have a laugh about it*

I really don't see your friend just randomly announcing it and the other boy just saying "oh, really? cool." as that's just not how school students work. He'd have only brought it up to "gossip" about you in a negative manner.

So I personally would forget they ever existed, and try to make closer friends elsewhere. Once school is over, a lot of people usually end up with a completely new group of friends via work/college etc anyway. The chances are these people would have never stayed in your life in the first place, whether they were half decent friends or not, so perhaps it's no great loss.