Actually, it's not OSSAA doing that, they just give the option. Its Union taking that option. A decade, maybe longer, they put into effect a rule that your sport practice could be used as a p.e. credit. The idea was that it would slow down overloading of kids. Rather than taking their classes, getting off school, then practicing, and getting home, eating, homework, they tried to wrap it more into the schools by making it a class. They use to also offer it as first hour or zero hour option as well. Wrestling and basketball coaches seemed to like the morning option.
The big high schools went to it right away, it also allows them to keep coaches as "teachers" rather than having to move them into administration or limited rolls. I don't think they ever made it mandatory by the state, not sure if there are any of even the smaller schools not doing it though.