Firstly I think it would be worthwhile signing up with an agent that does casting for TV commercials. You may have to pay for a photo portfolio. Casting agents are always looking for extras. It's a good way to get some experience.

... also look around for an amateur theatre group or take drama classes. If you are serious about acting you'll need lessons.

Participate in school plays.  Borrow a video camera and make your own short movie...  with you performing in it.

Practice on your own in front of a mirror. Imitate accents and experiment with different ways to say the same set of words.

You'll need to be very dedicated and perhaps a bit lucky to succeed... talent, personality, looks, and social skills will help as well.

I think you should go to your school conciller and ask for his advice.You sound a bit upset.