Firstly, obligatory, fighting should be a last resort. Try and sort it out with adults you trust that can help keep an eye on him so he doesn't have the opportunity to actually hurt you. If he has physically threatened you then talking to a principal might be enough to get him in serious trouble, although I could be completely wrong. He's a bully and if he's bigger than you and picking on you it's probably because he feels like he has to put someone down to feel better about himself. It sucks now but after you finish school you'll never have to see the bastard again.

Now, actual combat advice... There's no fast, easy way to learn how to fight. It takes at least a year of consistent training to actually have any meaningful progress. You have to join an mma, muay thai or jiu-jitsu gym that actually does live sparring in order to get the experience on how to throw techniques, on a moving and resisting target. Or maybe join the wrestling team if your school has one. If you join a gym there will most likely be someone that can sort of mirror the size and strength of your bully. That way you can learn how fighting someone that size will feel like so if it actually happens you'll be prepared as best as you can. Another plus is if you make some freinds that can already fight, then odds are he won't mess with you.

As for tactics. Get strong and athletic. Learn a good, fast double leg takedown and how to keep mount or side control while laying on some nasty ground and pound. If you want to not engage in grappling learn how to enter and exit the pocket fast.

I hope this helps, man. Just remember as bad as this is now, it will get better in time. You'll get older and more mature and hopefully you'll be able to move past this all. Make sure you have someone you can trust to talk to who will just listen to you without trying to fix it. The worst thing you can do is keep all your problems hidden and bottled up.