Schooling fish will no longer school once they feel comfortable in their surroundings. Schooling is a defense against predatory fish. You can certainly keep a few Tetras. I've kept one Tetra in a 45 gallon tank for more than a year along with one Zebra Danio. Both are in a Community tank that includes other Tetras, Guppies and some Comet Goldfish.
As long as you keep the water clear of dissolved waste material, the fish will remain healthy.
"Fear not, my young apprentice. Just change the tank water."