I appreciate these ideas and input. I knew heating would be a significant challenge. I was wondering if multiple small heaters plumbed in series would get the job done, but at the rate calculated above it would take several of them to be close to practical.
Electrical bonding. I hadn't thought of that! This is why I posted here.
What about filling with hot water (i.e. from the school's hot water system)? As long as your fill water works out to close to your target temperature (which I would guess is in the mid 70s) the smaller heater just has to maintain - much easier with an indoor pool and an indoor environment of 70F - the delta T isn't bad, and the lowest temperature the water would get without heating is room temperature.