Wait a minute! I know that parents aren't always logical, but ... if your parents aren't convinced that you are responsible, and they don't trust people on the internet, then why are they permitting you to use these sort of sites?
Did I understand correctly that a pipefish was recommended for a classroom aquarium? Pipefish also need a lot of care. They aren't the type of fish that I could see leaving for holidays in a classroom...
Do you have an aquarium already? If so, what size is it?
Both you and your parents should do some research this summer. The number one thing that should be done: go to the LFS when they get their fish shipment in. If they are like most shops, they open the boxes on the floor and both you and your mother can get a realistic picture of fish mortality when fish are shipped from wholesellers. This will also give her the idea that wholesellers ship with price (i.e. weight) in mind, not the welfare of the fish. I was shocked when I saw freshwater betta fish shipped in tube-like bags with barely an inch of water in them -- in fact, at first I thought the fish were in the bags because they were dead, as it was just a hundred little bags with air all thrown inside one large garbage bag. The kid who was about to unpack them told me that this was actually better shipping conditions than most bettas receive -- often they are shipped on wet paper towels.
Next, if you are still interested in dwarf horses, there is a book that was written all about their care. You may even be able to find it in your local library. Perhaps both you and your parents should read it. It covers everything you'll need to know about successfully keeping dwarves. Of course, my copy isn't at this house and I don't remember the author's name off hand, but I think it is the only dwarf seahorse book published in the last decade.
If you have a smaller tank and would like a similar experience, but one that would be a bit easier, I would look into getting a single blue stripe pipefish. Although the pipe would likely need the addition of freshly hatched brine shrimp (which you would need to hatch, just like for the dwarves), the pipe can and will hunt down the brine and will appreciate live rock in its tank (which will help with filtration.) So you would still have the responsibility of the food (which is a major part of owning dwarves), but some of the husbandry issues would be easier, and there is a chance that you could get the pipe onto frozen foods. Plus, the pipe could live with other peaceful fish -- perhaps a goby or two.
Good luck.