It is a tough call, what to teach or tell someone, somebody said above basically you MUST use any training or weapon RESPONSIBLY. Getting picked on seems to be an important part of escalted violence in school. In the 4th grade a kid was found to have a razor blade on the bus because some kids were picking on him.
I don't know if this was answered but most "Heavy bags" should have a place to put a rope or chain on the bottom also so if you don't have anybody to hold the bag, you can attach it to the floor, or to plates from a weight set. Boxing is a good discipline, but the longer a "streetfight" lasts the more likely it is on the ground. Wrestling if a sport at your school is a good start. It should be inexpensive for you if that is an issue, you will probably need a sports physical, and they teach speed, power, and strength, I know that power and strength seem similar, but they are different. They should be able to teach you to "shoot" on somebody basically put him on his stomach while you are on your knees and give you time to escape. Unless the person hits their head on something hard like even a tile type floor or the corner of the desk, which happened in a one punch fist fight at a highschool and the kid died, it may be the cheapest, and quickest bang for your buck. If you can before wrestling season starts get a month or two of Martial arts training it will help.
I would not try any type of "killer move", instep stomps if some is bearhugging you, or throwing your head backwards to hit their nose and just coming forward and stepping on their toes and pushing or the nose punch, the basic stuff said above all works. If being puched at hands up elbows close to the body, and keep your chin as close to your chest. I never attempted a kick above the knee because I could not pull it off, and getting your foot grabbed and landing on the ground was not for me. Learning your limititations is a very good thing. If you are not fast but strong you can train that way and work on your weakness, and soon it becomes your strength, and you keep doing that, and you will progress in most anything in life.
To strengten wrists and forearms if you have dumbells sit put your forearm on your leg your wrist over your kneeat least or and relax your wrist and then pull your hand up. Do it with your inside of your forearm also. Alot of this stuff I need a pic to visualise. Search on wrist curls, reverse wrist curls. Grasp a tennis ball tightly and relaxing also helps grip strength. Me and my brother got exercise books from the library. There is a trend in equipmentless training to keep costs low. They may have some self defense books, that may help with footwork. You can try a bookstore for some books, I am trying to keep costs low so you can spend it on ammo for target/plinking, or whatever. Good luck!
I guess when we sparred like that we were sissys. We had Headguards, Martial arts gloves, mouthpieces, and footguards.