Actually this is very common questionhow can I want to convince my girlfriend parents to accept as there boy for whom they are looking for a girl.
My girlfriend is under very tensed position because her parents are not accepting me as rite choice for her

I don't think verbally convincing them is the answer. SHOW them that you care for their daughter. And that you want her to be involved with her family. Make sure she is home by her curfew. No weekday dates unless there's no school. Don't go into her bedroom when parents aren't home. Respect their rules. Say yes ma'am, no ma'am, yes sir, no sir. ALWAYS ALWAYS ALWAYS greet the parents and ask how their day is.

Small gestures go a long way.

Another thing is studying together. I know that sounds silly and trivial. But remember, it wasn't too long ago that I was in high school. Parents love the fact that you are encouraging their daughter to do good in school. They see that its also a top priority for you. And that if she needs help you are there...

Had to spread the rep, CHM, but those are excellent suggestions. We parents react to actions not words.