Insulating a garage

[restew1017]

Hey there! I'm an acrobatics teacher currently teaching out of an old tire shop/garage. It's an old, uninsulated cinderblock structure, and it's about to get way too cold for me to continue teaching there. The ceilings are about 15 feet high and there's about 800 square feet of exterior-facing wall to insulate. There's also an open hallway connecting to some unused, unheated space that I'd like to close off.

What would you recommend as far as material and method? I'm leaning towards rigid insulation, extruded polystyrene; my carpentry skills aren't that fantastic, though, so any way to install that without a frame would be much appreciated.

Thanks!