I really hope you can sort this out soon.
My son, almost 15 now, has been bullied in school since around Primary 6 onwards, but it's mostly been girls doing the bullying. Physical bullying has happened, and he's not really prone to fighting back, but when he has that's stopped, but for the other stuff, I've tried to teach him that a "thicker skin" is ultimately required because there's next to nothing the school can do about verbal abuse, but I never developed one so I have no idea how anyone does that when it's not natural to them.
To be honest I think girls can be far worse bullies than boys simply because it mostly is on the verbal/psychological level. I was bullied for a while when I was 9 and it was my first year at an all-girls private school. I actually begged my mother to take me away but all she said was that it was time I learnt to stand on my own two feet. Fortunately my tormentor left not long after, and that was that.
As for developing a 'thicker skin', I can relate to that as well because I didn't have one either when I was younger, but fortunately as I got older, and by that I mean 40+, it just sort of developed of its own accord, although I don't know why - maybe due to an accumulation of life experiences? - but I agree that it certainly isn't something that anyone can be taught.