Balas won't school

a "school" is more than two ... two is a pair. bala sharks grow to a foot and should be kept in groups of at least 4. a 40 gallon is too small for 4 bala's. you need a 75 gallon tank for bala's when they're small, and at least 150 gallons when they're larger.

BTW, some fish will only school if they feel there is a danger to them.

As others have said, you need around 5 to see the proper schooling that you are looking for. Also a huge tank is required for balas and they usually will not school unless there is a dangerous fish in there. Also fish that do school usually will not unless the aquarium is pretty big, like you dont even see much tetra schooling in a 20g but if they were in a 75g with the same fish and maybe something bigger that they were a little afraid of they would do it