This one happened at the same school I attended back in the early 80's.
www.youtube.com/watch?v=_UaEBFJOlgY
Signal 21 is a county wide code for "Officer needs Help!" As a Deputy we used Signal 44 to state the same thing. In fact, our Radio room includes dispatchers for Akron PD. We dispatched for ourselves and the County Park service, along with the ODNR in Summit county.