Ahoy there!
I was also bullied in elementary school, high school and college. During PhD, I've also been bullied once, yet I'm unsure if that can be called bullying since it was from a teacher rather than a classmate. It was usually more verbal than physical, despite there having been both kinds of abuse towards me.
The "cause"? I talk too much, prefer to stay alone, the smart kid too (maths FTW), don't share their interests, moved in from another town, I'm "weird" and even once they hunted me for a year because I didn't want to give the bully-in-chief my rubber for free.
The real cause? I'm sure some of them had a tragic backstory, yet most of them were because of acceptance. I was different, I wasn't in their group so the group built frontiers to not let me in. Just remember that you are not the cause (unless you go to them and insult them or other explicit provocation means, I've known people that hit first and then they hide their hand. I know you are not one of these people).
The solutions I took? For the verbal abuse, I just "ignored" them. My father once told me: "If what they say about you is a lie, ¿why bother getting angry about something you know it's untrue? Otherwise, if what they say about you it's true, accept it for it is a part of you, try to change it if you don't like it and move on". My mother also told me before entering high school "if they say something about me, you know it's not true and whatever they say I won't know, so don't defend my honour or something like that, just ignore them", which comes handy for all mother related ways to mess with people verbally.
It's not a passive way of ignoring people like looking away and not answering when they tell me something. Is more like actively ignoring what they say. "Eat a burger for one" -> "Sorry, I'm a vegetarian". "Say something" -> "Something". "You're ugly" -> "You're handsome". The last answer left a bully of mine confused for more than one minute, the time I used to leave. In the end, I answer casually to stuff they say to mock me, sometimes even smiling at them which confuses them even more. They eventually get tired and look for another victim.
For the physical abuse, I've always avoided any situations in which I'd be at a disadvantage. Despite the teachers not doing a lot about bullying in my school/high school, if they saw people fighting they'd stop them: I always had a teacher located, to get close to him/her in case one of the aggressive bullies came too close to me. I also learned Judo and my father also taught me some other personal defence stuff. I'm glad to say that I only needed to make use of that thrice.
Finally, I'm sad to read that your friends don't support you. Have you already talked to them about your feelings? Try to do it by speaking about your feelings: how do you feel, what makes you feel like that, what would you want them to do. Avoid saying "you never do something" or "you always do something". Just state the facts. Bad example: "You always take my stuff without asking". Good example: "Yesterday afternoon you took my pencil without asking. I got worried because I couldn't find it until I realized you had it. Could you please try to tell me when you need something so I don't think I've lost it?". Yes, it's larger, yet it can help you ask someone to modify their actions. These are called I-messages.
I hope some of my anecdotes/quotes/advices/jokes help you.
You are not alone, feel free to contact me whenever you want. Just remember I'm not very active so there might be a big delay in my answers xD.
Mowstyl.
