Associate yourself with a good local dive shop and then talk to them. At our shop we have two highschools signed up. There are many insurance programs which will protect the highschool. I would be surprised if your high school doesn't already have one in place.
Now the other consideration is that dive shops are in business to sell gear. Like college students most high school kids don't have money. So of course they'll always want it cheap and seldom will the purchase locally when they do have money because most move away as adults.
Recently I had to pull one high schooler aside and mention that if he contined to walk around talking about all the "great" bargains for fins and stuff online that we would have zero reason to continue our relationship, at least let us sell stuff to our regular customers
Good luck.
Now the other consideration is that dive shops are in business to sell gear. Like college students most high school kids don't have money. So of course they'll always want it cheap and seldom will the purchase locally when they do have money because most move away as adults.
Recently I had to pull one high schooler aside and mention that if he contined to walk around talking about all the "great" bargains for fins and stuff online that we would have zero reason to continue our relationship, at least let us sell stuff to our regular customers

Good luck.