well it sounds like you had no objections to him being in the play until you found out it clashed with the changed football game and that the teachers were telling you what to do

at the end of the day if you break your promise, and take him out of the play, you will be upsetting the teachers, who may hold it against you in the future. if your son is 8 he has a few more years at that school, and the teachers could make it awkward for both you and him for the remainder, is your son's education worth risking over a game of football?

you made your comitment to the play, it seems very unfair to pull out last minute because of another changed comitment.

but at the end of the day, the choice is yours, but either way someone's gonna get upset.