Things have changed so much from the time I was in middle school, yet some things tend to stay the same. It is a bridge between elementary when children often have one primary teacher to having multiple teachers for different subjects. It is an excellent time to participate in school sports to test your interest and talent level. Some sports require tryout while others allow everyone who is interested to participate.
There are academic requirements at some middle schools. For example, in order to play the student needs to have a "C" or higher to participate in games. The idea is to keep academics first and the other benefit is that smarter and disciplined students can achieve more on the field.