Bumped into a parked car

Unless you are able to find that car again there is nothing you can do.

Nothing we can do, you either call the police/insurance and tell them you hit someone and don't know who, or you wait and see if the police come and grab you.

Will I get a criminal conviction if I do this?

First option No, Second option Yes (potentially)

In the case of very small scrapes it is plausible to say you weren't sure/didn't notice till you got home, Y'know, as long as no one/CCTV saw you clearly get out your car, look at the damage shrug and drive away again, and as long as they didn't have any dashcams you tripped... see the message here is you always try and tell the owner or leave a sensible note as you never quite know who is watching.

Yeah I know I just freaked out and left I was just so scared, I didn’t get out of my car to look at it, although I think I opened the door to have a look at it, I didn’t notice the scrape on my car until I drove off and got out and looked at it. It’s a school car park. What will happen if I call up my insurance company or the police?

Honestly unlikely they’ll follow up on it, especially if it’s only a little scratch. I once reversed into someone’s car by accident and drove off, never got done for it but there wasn’t any damage tbh. None at all. And I’ve also driven through plenty of lights that were on amber and turned red, I had enough time to stop but didn’t. Never got any points for it. Tbh, people do worse things and don’t get done for it, makes me wonder if traffic cameras are actually even real or just deterrents...

I can't say for sure, I've never been in this position, the one time I had to call/claim it was v v obvious it was me that had crashed the car as it was wrecked into a traffic light, I couldn't really claim I hadn't noticed.

A school car park most likely will have CCTV of some sort, no way of knowing if the other owner has noticed/complained/doesn't care but the first thing I'd be doing is leaving a note with the school reception. Until you are able to contact the other driver you are obligated to inform the police/insurer, but without the other car/drivers details there really isn't much for them to go on beyond making a note of a potential future claim. Expect some pointed queries about why you didn't take details at the time though, but if you're the one calling them they won't get too mad. (this is somewhat guesswork on my part)