I am assuming that you, like me, are only miking vocals. Keeping over-all level down is a good idea. Drummers often have trouble doing this in a large space, so you may need to remind your drummer several times right at first.
A typical US High School gym is often at least 60 W x 100 L x 40 H (feet) with concrete block walls and pilasters, steel rafters and ceiling, and shiny hardwood floors. In short, a giant echo chamber.
Crowd size and noise, as well as overall size and reflectivity of the space are going to determine exactly how much reverberation you are dealing with. Needless to say, using reverb effects is pointless. Players will want to crank everything. Don't let them.
Note, I am not worrying about reaching higher volumes or fighting feedback as much as I am trying to help the band hear itself in a sea of reflections.
I have observed that keeping players, amps, and PA speakers closer together than usual helps. I believe this is because it puts each player in about the same reflection environment. We are using stage volume as both monitors for instruments and FOH.
Certainly going the Arena route and having low stage volume and everything through a main FOH system is a good approach, though I haven't the gear to do this well.
I may experiment with sending the stage mics to the gym's installed system at the next event. However, we generally work with just 2 mains and 2 monitors.
