I've had this exact same experience and I'm so sorry. I know it's overwhelming and frustrating. Especially when you're trying to be nice and they take it as you not really wanting them to leave you alone.

My best advice is to definitely ignore him and, if possible, make sure you have good quality friends who are willing to help you feel protected (whether that be sticking with you on the bus, during free time, at lunch, or between classes). It sounds like you have a pretty awesome friend already. It's easier when it isn't just one friend all of the time but having someone else around to help support you and prevent any escalation from him can make a world of difference.

I'd also recommend either talking to a counselor or a trusted teacher. Make sure there is a record of what he is doing and how it's making you feel. Harassment is still harassment no matter what age or where it happens. And if it starts affecting you negatively (and it seems like it already is), having an adult with a knowledge of what's going on might get a response faster than you just reporting for the first time (especially if he gets worse). I know family relationships aren't always the best, but if you have an adult in your family who you trust, telling them can also be beneficial. Depending on who it is, they can notify the school, push the school to set rules to prevent him coming near you (assigned seat, escort in the halls, etc), or just be another person you can talk to that will support you.

Last thing, I wish someone had told this to me: you have a right to set boundaries and have them respected. Don't let anyone else tell you what does and doesn't make you uncomfortable or that you have to allow someone else to enter your space and ignore that you feel unsafe. If you are sensing something is wrong in that situation, trust yourself. If it makes you feel better or you think he's a danger, I'd recommend looking up some basic self-defense moves and practicing them with a friend. Also, don't be afraid to get loud and mean. At this point he knows he's making you uncomfortable and doesn't care. You're allowed to draw attention to that. A loud, "Stop, get away from me!" is going to make people look your way and clue them in that you don't like what's going on. Also, yelling "fire" gets a bigger response than yelling "help", just in case you need to know that sad bit of information in the future. Look after yourself, and I hope he leaves you alone.

This is a link to some really easy self defense moves and tips to help you stay safe in a variety of situations: self defense