Who's making your life a living shit? Do people at school know that you're bisexual?
You're only 18! Does it really matter right now what you call yourself? Life's too short to be stressed about something as lame as a label.
Don't worry about it. Gay, Bi and Straight are all just labels.
It sounds like at the moment, guys turn you on. Do girls turn you on too? If so, then cool. If not, then cool.
Do you have to tell others right now? Only if it's really bothering you.
If you're Bi, you still might feel the need to tell people. So if you're worried about whether you're gay or bi, it probably doesn't matter too much at this moment, except that if you still like women, you'll be able to not tell people (that you're bi) and still have sex with women without lying about it. Meaning you can not tell people and still get off at least.
If you do decide that you're gay, and it's really really bothering you, start by telling someone you are pretty sure will be accepting, understanding and discreet about it. You can then decide who to tell next, and figure out how you feel about telling people. It gets easier with everyone that you tell, for you at least.
This is really just a small part of your life, and is only as big of a deal as you want to make it. If you do start feeling comfortable about it, and don't want to make a big deal of it, you could just not come out to people directly, and you could just not hide it when you're checking out other guys. People will notice if they notice, and won't if they don't. Either way, it saves you having to sit down with them and have a big heart to heart about it. Of course, the heart to heart can be really good too.
Good luck either way. Just don't waste time stressing about it.