A bully is after me and i want him to stop?

im 15 its summer this guy is a year younger than me but his stronger and he has lots of people to go after me but so far its only him . the other day i was alone and he chased me but i got away he wanted to fight me for no reason he is also after my friend. i want to forget all of this. and told my girlfriend and she got involved and it seems things got worse please help