Considering everybody masturbates, especially boys, I'd say they're just fascinated that you admitted it. It'll pass. Your boyfriend is a jerk for telling them, and you should tell him so: he shouldn't have betrayed your confidence, shape up, or ship out. But you're both young, and this is just the sort of crap that happens in high school. Just remember to not be ashamed of it, like you said, it's natural. And even if they try and tease you on it, ask them what they wank off to, join in the joke, instead of letting them make you the butt of one.

Yes, you have the right to be upset. You told your boyfriend in confidence, and he told his friends. As cruddy as it is, this is pretty common stuff that happens in high school. I'm really sorry, but it'll pass.

Sweetheart, your boyfriend sounds very immature. You have ever right to be upset. However, the damage is done now. It would probably be best to break up with him and just say he is a total liar. It is natural and its your body, so you have nothing to be ashamed of! But, it's still no one else's business!