[Dabhand16]

Depending how big the group will be, one flash might not give a wide enough coverage. Might be better to arrange them deeper and narrower - back row standing on benches - next row standing on floor/ground - next row sitting on benches - front row sitting on floor/ground, or use two flashguns.

If you are going to use a flash, get hold of it well in advance and check the effective range and guide number. You can have a practice using half a dozen volunteers - one in each corner and two in the middle (front row and back row). Then you will see what its coverage is.