How do I find a job that actually uses what I learned in school?

What you'll find is that most positions—whether in academia or in industry—you will not be directly applying most of the skills you learned in school.

As a "knowledge professional," the most important "skill" you'll use is learning how to learn: you'll likely be placed in situations where you have to figure out what you need to solve, and how to come up with a solution. Your background will provide help in figuring out the problem—after all, the principles are the same—but you'll typically have to simplify your analysis (because of time or resource constraints, and so on). Or you might have to branch out into new areas that you haven't learned before: being able to do so quickly will give you a big advantage over the "competition" (whoever or whatever that may be).

If you haven't looked into cryptography, you should. It requires a lot of complicated math along with computer science, and it pays quite well.