Its poor water quality, most likely ammonia from the fish living in its own waste.
There is a natural process called the nitrogen cycle that removes waste from aquarium, but it takes months to establish. Until the cycle establishes the aquarium needs a little help from you doing a lot of water changes to keep the water clean and safe.
Keeping the fish in a small container isnt going to help. Less water means less dilution of the toxic waste the fish produces. The fish needs to be in a proper sized aquarium, with a proper sized filter. And that aquarium needs to be cycled. The tank needs to be big enough to keep that lone fish and some more of the same species, so the aquarium needs to be 20 gallons/ 80 litres in capacity. And it will need a filter, a heater, and preferably a light.
Did the school just give you the fish or did they give you equipment to keep the fish in?
If they gave you nothing to keep the fish in, are you prepared to invest in providing the fish with a healthy environment? It will cost some money, or maybe you can look at the used aquarium market. If not, take the fish back to the school.
In the meantime, change the water more frequently. I would be doing a complete water change 2 or 3 times per day in a small unfiltered container. Make sure you use water conditioner every time you change the water. Do you know what a water conditioner is?
What on earth is a school doing sending children home with fish and giving them no instruction on how to properly look after them? Its called animal abuse and the school should be reported to whatever animal welfare organisations exist in your country.