I am sorry to say this, but you can not keep fish in that tiny of a tank. A few kinds of fish can be kept in a 5 gallon tank, most need at least a 10 gallon. A 1 gallon cannot be heated, which is probably one of the reasons why the Guppies died. They do not care about the pH of the water (most fish really don't), but they do need nice warm water, upper 70s F. Also a tank that tiny can probably not be filtered, which is necesary for fish. Do you know about the
nitrogen cycle? If not read the Fishlore Articles for Beginners, at the top of the freshwater beginners page.
Even if there were fish you could keep in that tiny of a tank, I would still strongly urge you not to get any. A school is a very stressful environment for any animals. There is so much noise and commotion, not to mention possible kids tapping on the glass and generally bothering the fish, they would be extremely stressed out.
Sorry I can't sound very optimistic. but you really can't humanely keep fish in that small of a tank. There are a few invertabrates you could maybe keep in a 1 gallon tank (they could LIVE in there, but not thrive), but I am very against any animals being kept in a classroom.
Don't be discouraged from keeping fish though. It is a wonderful, rewarding and--I'll admit it--addicting hobby. Perhaps you could set up a tank at home. Before you do, though, do a ton of research and ask lots of questions. Fish keeping can be difficult and requires a lot of knowledge.
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