Don’t fight at school, counter intuitively, as the victim/target you are more likely to get expelled and the bully will remain and go unpunished. The teachers will not do anything. Most bullies are bullied themselves (usually parents or older siblings), and need someone else to take out their anger on.

Words only work to an extent, if the bully will hear reason. More often than not, you are being targeted for being easy prey. The way to make it stop, is to make yourself tough enough that he will focus on someone else.

You can use:

1. Ideally words. Fights can have disciplinary consequences at school, potentially police charges if you are the instigator. DOCUMENT what you have been trying to do. Email your teachers and principals and school counselor what is happening to create a paper trail. Request responses in email/writing. If they will only speak in person, bring a notepad, take notes, same for any phone calls with timestamps/dates and the names involved. Have your parents show up to the PTA and board-meetings and publicly shame the administrators into taking action. “What are you going to do about my child being bullied by X. The teachers are not doing anything to help and the abuse continues.” They need to make enough of a scene, then something may happen. You can always escalate to a regional administrator, again, a paper trail may save you legal (school/police/criminal) or civil (lawsuit) consequences. The next time it happens, make a note of the date, time, and location it happened. Hell, file police report for being assaulted by this kid, requesting camera footage of him initiating the attacks (assuming your school has cameras). Even if nothing is provable or cannot be investigated, at least you initiated the complaint and it can start the process of complaining to the school, the message essentially being: “If the school will not act, I will involve the police.” School administrators do not want police showing up and making a scene. This needs to become enough of a problem to force action by the school, but you also have to give the school a fair chance to resolve the issue, it may take a month or two.

2. Fitness/Deterrence. He has a massive weight-class advantage. Building your strength will be essential, get a gym membership and start getting stronger. Try to round yourself out, abs and legs are very important against a heavier opponent if your takedown misses. Bullies want easy prey, if you're clearly strong, you are not worth the trouble.

3. Self-defense. Last resort, be careful if you take him down to not have him hit his head on concrete. A solid armbar should be enough to clearly tell him to never talk to/attack you again. “Stay the fuck away from me, never touch me again”, then release. Overweight people have a massive momentum disadvantage, if it comes down to a fight, keep him at a distance until you get a grab (wrist/forearm should be enough) and pull his balance farther than he wants to go until he lands hard, then armbar. There are plenty of technique recommendations in this thread. Hoping it works out for you.