talk to a school counselor.
and then what? your parents talked them out of helping you?
hmm, start setting fires. [jk]
Any teacher at school you can talk to without him/her telling your parents right away? As in, not all your deepest secrets, just, tell him/her you want to talk to someone and you don't want your parents to know about it. See if that is possible? Should be some teachers you can trust? Maybe the mom or dad of one of your friends? Maybe they can help you.<br />
Also, in Netherlands we have something called 'kindertelefoon'. It means 'kids phone'. Basically a phonenumber you can call if you feel you want to talk to someone. It's anonymous and an adult will listen to your problems and see if they can do something, point you in the right direction. See if you have something similar over there.
Do something a little nutty not insane, just a little off balanced at school, or in front of a teacher, so to convince your parents you need some help.
sadly i do no think you can unless you were banker acted by an authority figure...... principle, teacher at school, or a police officer,
What do you think is wrong with yourself