What will happen if you drain an inground pool?

Depends on the type of pool and location. A concrete or fiberglass pool should have much problem temporarily, in norther climates or in an area with high water tables both could "float" basically the frost or high water could push the shell out of the ground, in some cases ever a few feet. This is a huge problem, with a massive cost to fix. Vinyl Liner pools don't have exactly that same risk but without the weight of the water holding the liner in place it can shift and/or crack (for older liner) and will no longer be useful. I'd advise against it, it is a bit unsafe as well, someone could get hurt falling in!