So… have them take the ACT on their own? If they are low income and the fee is the problem, fee waivers may be available.
It does seem weird to switch in the middle of the year though.
Most test takers don’t actually study for standardized tests so I’m not sure that this is going to be any sort of big problem. Unless, of course, the teachers were “teaching to the test” for the ACT… in which case, there are a whole lot of other problems.