Platy Pregnant Or Poorly? Plus Nasty Guppy

Are you checking your ammonia and nitrite levels? There are some bacteria on the substrate and you might well have killed them when you washed the gravel under the tap. But there would not have been nearly as many bacteria as in the filter and they should have made up any losses very quickly.
How far, in terms of tapwater chemistry, was the move? The same town should be the same tapwater but if it was further, could your home and the classroom have different tapwater?

Guppies to have this unpleasant habit of fighting each other. With several, the nippiness is spread out but with two, the dominant one could well be picking on the subservient one. I'm afraid that's just the way guppies are.
Fish are always hungry. They don't find food every day in the wild so their instincts are to stuff themselves when they find food. They don't know you'll be feeding them every day. I would put the scrappiness down to to the just being male guppies rather than not being fed enough.
If you were to get some female guppies, you would need at least four (two females for every male) which would result in four batches of fry every month. And guppy fry are not the easiest of fish to rehome as they are so cheap. Shops are reluctant to take them as they can get so many cheap fish from their wholesaler so why risk possible disease from an unknown tank?

The platy - if it is female, it will be pregnant if there were males in the same tank in the shop. Pregnant females do tend to go off on there own just before they drop fry, and can be quite grumpy with fish that go near. But being fat could also be a sign of dropsy, and if it is male it's not pregnant. I know it's hard to see it, but if you could manage to look down on it from above you'd be able to see if the scales are sticking out from the body.

Whenever I've needed to clean the substrate I've always washed it under the outside tap, but checked the ammonia/nitrite levels for a few days to be safe and never seen a blip. I've only cleaned it like this when I've moved a tank and had to empty all the water.

Sparkle (platy) has sadly departed to the big fish bowl in the sky  . Turns out he was a boy after all. I've had a good look at the corpse and can't see any odd colours, redness, roughness of scales. The other fish are looking fine so far. It's just odd that two fish of different species have both died within a day of being put in the tank. I am worried that there is some bug in there and I am sentencing the rest of them to death by not doing anything!

There's a lot to be said about a tank with corys at the bottom, barbs in the lower half and tetras/rasboras in the upper half - or something similar. Peaceful, colourful and robust!

Great idea - I'm going to investigate some good options for the next tank!