My DS2 was bullied mercilessly at his first school.
He wasn't at his second, but he did have a tendency to assume that if he fell out with someone, they would hate him for ever.
And this came from his experiences at the first school.
It is all about self-confidence, and feelings of self-worth.
And the second school were excellent at appreciating that he was very socially vulnerable at first, and helped him to make friends.
We were told to take him for martial arts lessons, but for him, it wasn't about being able to defend himself from aggression, more being proud of being who he is.
So we bought him a guitar, and he writes and sings his own songs.
And he's now got a few 'groupies' , who think he's rather cool - and so does he!