It's inside the school on the downstairs level. I don't know where exactly but drag you cursor over the lockers and you'll find it.
It is in the school.
you look for the locker that is different then the others :-D
Open Greg's locker.
Open Greg's locker.
you go to greggs locker
5th lockers from the broken locker go to the right
His locker combination is 9-37-15.
In Greg's locker>
you cant.
9 37 15
type in 9 37 19
Figure it out on youtube and do it yourself