"mathematics" may end in "s", but that doesn't make it plural. We say "Mathematics is a science" not "Mathematics are sciences" (replace with "art/arts" if you don't belive it's a science). We also say "mathematics is a growing field", not "mathematics are growing fields". Because it's singular, the fact that it ends in "s" doesn't naturally require shortened forms of it to carry the last letter at the end. I get that in the UK people say "maths", but let's not make this out to be like one side is correct and the other is not because mathematics ends in "s". Particularly as "math" was first coined in the U.K. (much of contemporary U.S. English is an older form of contemporary British English).