mathmatics

Hogwarts is a magical school, so you don't find the same classes as Muggle schools there, as all the subjects taught at Hogwarts are related to magic. Also there is no maths class in the original series (books & movies). I therefore believe teaching a maths class in Hogwarts wouldn't be realistic.

but knowing math might open more doors for other jobs.

Which would be Muggle jobs ^^ Very few wizards choose to do a Muggle job after graduating, and most of those who do use to cheat a bit, for example Kingsley Shacklebolt uses magic to have his work done without effort when he worked at the Muggle Ministry. Mathematics can be solved very easily with a wand, if it's not the problem itself that is solved thanks to magic. And for these kinds of Muggle jobs that require special diplomas, wizards would either fake them or study in a Muggle school to have them, and therefore learn maths.

Arithmancy is on all sites (but one I believe) and it involves quite a lot of math in it, so this is already a plenty good substitute. A normal math class wouldn't be realistic, since its not in the original series, and there wouldn't be an entertaining way to have a professor teach this for 7 years to students who already take math offsite.