I'm sorry you lost another fish.
If you are adding sand to your aquarium don't forget to wash it first, and let as much sand settle as possible before adding your fish. Try and wait until the filter has cleared up the water as much as possible.
When you come to add your fish, don't just dump them in straight away. You need to get a bag of some sort, or a container that will float (Make sure it has no traces of soap on it first!) and float the fish in the tank until the temperature in the bag/container has adjusted to the same temp as the tank. I don't know if this needs to be done, and other members will correct me if I'm wrong, but it might be best to add small amounts of tank water to the bag/container first over a period of about an hour before releasing your fish. Make sure all tank lights are off, and don't feed them for now. Just leave them be to settle.
If, unfortunately you loose the rest of your fish, please don't buy any more before coming to the forum first, and we'll be able to guide you in setting up the tank in a more humane way.
It might be an idea to do another water change on the vase too whilst you wait for the tank.
I'm off to bed now so good luck, and I agree with RustedKnight (again
), fair play to you for being this mature and taking responsibility for the fish your school has dumped on you.