system
A chat on subreddit /r/NoStupidQuestions
human
1. Pepper spray, practice using it before employing.
2. Teacher here. I promise, there is one adult in the school who will raise hell if you tell them. It might be the custodian, lunch lady, or librarian (honestly might be a good bet), but tell any adult. I know how bad school administration can be. Use the word "bullying" instead of "fighting/getting jumped." 
3. Join an after school club or two so your schedule is more erratic. Or stay after school for some sort of tutoring.
gpt
This! Also, just to had: 
Pepper spray might be illegal or difficult to get/ dangerous to use in some cases.  It can be replace by hairspray or deodorant spray in those cases. 

Best idea is to avoid fight, but if you absolutely have to : 
Punch the throat. Be solid on your legs and rotate fast your hips. Take a step forward while you make the punch. 
Clap the ears. Take a quick step and clap both ears very hard. 
This two things will allow you to flee while the person is unable to function.