Math and maths are equally acceptable abbreviations of mathematics. The only difference is that math is preferred in the U.S. and Canada, and maths is preferred in the U.K., Australia, and most other English-speaking areas of the world.
Neither abbreviation is correct or incorrect. You may hear arguments for one being superior to the other, and there are logical cases for both sides. One could argue maths is better because mathematics ends in s, and one could argue math is better because mathematics is just a mass noun that happens to end in s. In any case, English usage is rarely guided by logic, and these usage idiosyncrasies are often arbitrary. If you were raised in a part of the world where people say maths, then maths is correct for you, and the same is of course true of math. Don’t listen to anyone who says otherwise.
Math is the strong suit of students at the Ward Elementary School, where 50 percent of third grade students scored “advanced.” [Boston Globe]
Math professors are appalled at the lack of math skills they see in some education students … [Winnipeg Free Press]
Apollo paid less than $100 million to acquire Carnegie Learning, a provider of computer-based math tutorials. [The Atlantic]
Outside North America
It lasted a long 40 minutes, which is how I remember maths lessons. [Financial Times (U.K.)]
But scratch below the surface and you’ll find the maths is seriously flawed. [Sydney Morning Herald]
The Government has been under pressure from business and employer groups to boost standards in maths. [Irish Times]