How can a 15 y.o. be charged with indecent exposure (a pretty stupid "crime" to be in the books in the first place) for that matter? As you state you have been given a police warning, which is nothing more than a telling-to by some zealot wearing a uniform. You have not commited an offence as far as the law is concerned.
And that's none of the school's business anyway, they're only asking because they want to sound all serious and professional for marketing reasons.
Hope that helps