I would see if the school can put it in a school newsletter (if they have one) or make a special note to go home. We had something similiar where the school made mention of it in a few newsletters, then stated that anyone caught doing the wrong thing, would have their rego number taken down and the police called..
I hate how the police 'patrol' our school just after 9am! They're not going to catch anyone doing anything wrong at that time! They need to be there a bit earlier than that, around 8.30ish i reckon..maybe if you write a letter, suggest a time that would be ideal, and maybe if they walk around as opposed to doing a drive by?