Well this is my 11th grade year I've always had a thing for lesbian. But its not straight girls its I have nothing to do with them. Well I have a girl in my health career class that is lesbian and I've talked to her all the time but it makes me feel weird and all shy and crap. Well through out the semester I've grown feeling for her but she has a girlfriend and I don't want to ruin that for her but see her girlfriend cheats on her all the time and it kills me to see her sad. I want to be with her.. But I'm scared to because I don't want people to think poorly of my decisions. So should I tell her I like her and see what's happens. Or should I wait a while and just let life live? I wouldn't be asking this stuff I'm just in a ditch and all confused about this because its something new I have discovered about myself.

First, you're not straight. You may be bi, but you're not straight if you want to have a lesbian relationship. So that part of the confusion should be cleared up. :)

Tell her how you feel, and if she shows interest, make it clear that you will not be someone she sees on the side. If she wants to be with you, then she has to break up with her current girlfriend. If she doesn't fell the same way about you, then at least you'll know where you stand.

There's no harm in telling someone how you feel, there's only harm if you try to break them up. Do you understand?

If you are pretty sure about your feelings just be honest and tell her out. May be this will become her choice right? At least it's better than no action and feel the pains. But what if she deny? Don't worry, just tell her, I'll be your best friend instead, then she may be OK with it. Or she'll fall for you someday. <3 hopefully.
Good luck loving girl :)