There are quite a lot of footballs on the ground around Bullworth Academy.
There's one outside the boy's dorms, outside the library, in the open space where the school bus with no wheels is (to name a few).
To kick a ball, just go up to one and press Square.
You can kick the same ball multiple times luckily so this will only take about 10-15 mins at most unless you want to do it bits at a time whenever you walk past a football, you can easily have this before the end of Chapter 1.
It's best to attempt this outside the boy's dorms or outside the library because they are more closed spaces than the other areas. You can even get multiple kicks in if you bounce if off the walls.
I'm not sure if keep ups will count towards this trophy at all because I never bothered with that activity at all, is anyone able to confirm this?