There are actually many areas you can go into with a Mathematics degree, beyond being a math teacher. It is a top scoring degree on both the MCAT and LSAT, and I've found that a Math degree shows employers a general baseline of intelligence for unrelated positions within a company as well. Essentially, Mathematics is viewed as an elite degree which shows more on your abilities than what you've actually learned.
But if you're truly a math focused individual, there are 2 main routes to choose from. The first is the actuarial profession (like me!). Actuaries work primarily in insurance and take a series of certification tests that effectively replace the need for graduate degrees as you progress through your career. Even those who struggle with exams can easily find a home in an actuarial related field within insurance companies or consulting groups. These companies typically hire individuals pursuing certification and will hang onto strong performers who can't pass the exams.
The other area is one you've likely thought of: academia. From theoretical to applied mathematics, you can even pursue advanced degrees in quantitative analytics. This path can lead to a rewarding career in research and teaching or can veer off into banking and investments once you've proven yourself by getting published or working your way up in an organization.
Either way, you're looking at a comfortable and financially rewarding career with consistent opportunities to try your hand elsewhere. Even for the undecided, I would recommend Mathematics just to keep as many doors open as possible.