Fair play for giving the school chance to issue an informal warning but I'd now report that to the police (highway obstruction causing safety issue near a school) and council (who probably collect the idiot-parking tax).
Yep, I'd report it that way too.
The school have no powers to stop this, all they can do is send out
yet another email to parents asking them to stop doing it because it's endangering children. I get these emails quite a lot as the parents at my school are fat lazy winkers chained to the car, but those responsible haven't got the self awareness / shame gene, otherwise the emails and the parking would abate for a bit. They don't.
I do overtly take pictures of the ones who park on the zig zags outside my school, (and there's one a week roughly). They often get out and confront me, it's the usual bullsh!t arguments and threats, "You don't have my permission to film".... "it's illegal to film kids, are you a paedo?".... "you're invading my privacy, if you don't stop I'll punch you".
I just calmly state that if they're back on those zig zags endangering the children on foot once more, it goes to the enforcement people. I don't see the same car twice. It's all they understand. Selfish gits like this go on being selfish if they're never challenged.