When I was a 12 year old boy, i stole some food on several days from the school canteen untill i was 14 (in different campuses). And as the school canteen people are on contract so they are seperate from the school so I can’t pay the school.
Now how do I repay as I dont know the canteen people. What do I do now?

You have to give sadaqah to the value of what you took with the niyyat of radd al madhalim, and ask for forgiveness.