I used to have that problem too when i was 13-14
What you need to do is portray a demonstartion of higher value (DHV)
Have a few stories handy for this.
At school it is almost impossible to run full game so instead, get the girl attracted enough for a second date out of school, on the weekends. This will give you enough isolation for the kill
Recap:
1) DHV
2) Be the alpha with all the right body languages/voice.
3) Keep the whole group involved while talking.
I suggest you read
http://www.gunwitch.com/seduction/gwm1.html9