Yes and no. It depends upon the meaning of "school."
Shoaling fish, which hatchetfish like all characins are, must be in groups of their own species. Numbers sometimes vary, but generally there is a minimum for a species but the more there are the better, always. This fish will be more relaxed, less stressed, and thus healthier. And frankly happy, if that is a state one can apply to fish.
And here we are talking the same species.
All species of hatchetfish will get along together, so they can be combined in the same aquarium, provided there are sufficient of each species and there is sufficient space.
The Marble Hatchetfish
is one species that always is better with more. I have had this species for many, many years, and I always consider 9 the absolute minimum, but 12 or more is still better. This fish is social, and in their natural habitat they live in groups of hundreds.
I happen to have some of the common Silver Hatchetfish
in with my marble hatchets; they rarely if ever interact. The silvers play among themselves, the marbles to the same. And they are in a 5-foot tank with plenty of space, yet they keep within their species.