Fix compilation error and sorting test in grade-school exercise

school.mli fails to compile unless I change the type of to_map to either use `Int.comparator_witness` or `string list Int.Map.t`; I chose the latter.

The test for the sort function doesn't actually fail on unsorted grades. I fixed that.

More details on my submission for the exercise: http://exercism.io/submissions/0a6074e20527cc9d425e2d04