Our school has drop off at the Kinder/1st grade playground before school. There are yard monitors watching.
However, what you should be teaching your son is how to deal with hard/awkward situations, not bailing him out every time something happens. I tell my DD that if someone is messing with her, she asks them to stop, and/or goes to find someone else to play with, or goes somewhere else. If the kid is still bothering her, then she is to tell an adult. The earlier you can teach your kids to be self sufficient & to stand up for themselves, the better. How is he supposed to learn if you are always intervening? You will not be able to save him from hard situations & you can't be there all the time, and teaching him how to handle himself is far more beneficial than getting upset every single time something like this happens to him.
Not to mention, you weren't there, you didn't see/hear the whole situation, and 5 year olds are notorious for exaggerating & making things up.