I'm wondering though if the problem is because you moved the fish from a dirty tank to a clean tank?

I'm sure unc;e has written round here that in a dirty tank, bad stuff builds up slowly and the fish may get used to it and survive for a while. If you move that fish to a clean tank or suddenly clean the dirty tank the difference in the water can kill. I think it was to do with osmotic pressure.

If you get another fish from that poor school tank, bring it home in it's dirty water and keep it in that doing small partial water changes every day for say a week. I'd do something like a 10% water change when I got it home, then another 10% the next day, and then maybe 20% for the next two days and so on. I don't know if that is right though.