After hour acces to school areas should be possible, especially if we chat up the right people. With 5-a-sides I gather you mean having a tournament, in a certain sport, where teams of 5 pay for the entry, and there is some kind of prize? That might just work, especially with the grounds we've got.
Battle of the bands, well, we haven't even got a big enough auditorium to put all the students in at once, so that might be an issue. But we could, in theory, build some kind of out door thingy, weather permmiting. We would need to get the stage hirer to support us by sponsoring.....