Yes, it is very much a real thing. Fighting during school hours would be difficult. Not only do you have staff and faculty around. The kids don’t have the same schedules. Especially in high school. Their schedules are spread out over multiple lunch periods and multiple classes. The only time you can guarantee for two people to be free of supervision and free to congregate without interference would be entering and exiting school. Before and after school. The only other time two student with different academic standings, different grade levels, different social groups, and different interests would be together would be during sporting events and class assemblies.