School fees are high and that is because fees is plural. As for the lumping together thing... there's no possible lumping here. School is an "adjective" here. Regarding thet phrase "Ten years is a long period of time". I am not entirely sure whether it is grammatical or not (Cassy?). Especially since I would write "Ten years constitue a long period of time" and not constitutes... mainly because the subject is "Ten years" which is, of course, plural.
In any case, you would lump "Ten years" together and consider it "a long period of time". In the sentence "Their school fees is high" you lump "school fees" together and consider it... what?