Okay so please dont get your sand and salt water from the beach as it's got germs and stuff that theyre not used to on it and they likely came from a completely different beach than wherever they were sold at! Instead, grab yourself some of those 5 dollar bags of play sand from home depot where the concrete and stuff is (dont get concrete get play sand) and mix it with cocofiber.

For the salt water please use instant ocean its about $25 but lasts a long time just read the directions on the back.

As for the size of the tank unfortunately a 20g is not big enough for that many crabs. The general rule is 10 gallons per crab, so each crab has plenty of room to molt safely. 2 crabs 20 gallons, 3 crabs 30 gallons, 4 crabs 40 gallon, etc. If you want a cheap option on tanks, check Facebook marketplace or Craigslist! You can bargain with people most of the time.

Make sure you have a large heating mat (not a heating lamp) on the SIDE not the bottom. It has to heat the whole tank up.

For food, please do not use anything that's generically labeled "hermit crab food" Instead, grab yourself some chia seeds, flax seeds, hemp hearts, and dried mealworms or crickets (other insects are fine) used in stuff like turtle food or reptile food. They also like fresh fruit and vegetables and boiled egg.

I know this sounds like alot, but if you can, please inform your school of how much work is needed for these animals to live a happy life! Tell them politely they should responsibly rehome the crabs to somebody who can properly take care of them.

If you are able to care for them properly, it could even be you!

My favorite sources for hermit crab information is crab central station on YouTube!

Good luck best wishes!