Yes, definitely check your local high schools. Lots of students need service hours to graduate. You can frame it as a way to get those service hours and have fun coaching!
Beware though... I've had issues with junior coaches because many of them aren't mature enough to understand how to interact with the athletes. Some of them try to befriend the older athletes, others speak out of hand, others are discouraging. Specifically, I had a high school alum help my middle school team for a few practices, and she was constantly questioning their ability to do the stunts I was teaching. I heard murmurings from athletes that she was saying things like "This is too hard" and "They'll never be able to do this." She didn't return after a few practices, but I was planning on asking her not to come back.
Just set your expectations from the beginning and be sure to communicate with the junior coaches to praise them for a job well done, as well as correct them when they're doing something incorrect.
