[TOMT] A book about a boy in a magic school who struggles and finds out he has anti magic

I read this a long time ago and basically it’s a book where a boy finds out he isn’t good at magic because of his inherent ability to wield anti magic. Him and his friends sneak around school to try find more information about his history, while hiding there motives from the evil headmistress. They find out that there was a war between magic people and anti magic people and that the antimagic people got all killed, he is the last survivor from that race.