What should be in a CyberSecurity Course in secondary school?
The course that I offered was all about gdb, hexdumps, buffer overflows, and format string vulnerabilities.  As a textbook, we used Hacking, by Jon Erickson.
 AP CS A was a prerequisite of the course, though I permitted students to join the class if they were currently enrolled in that course, as long as we had a discussion outlining expectations first.  There were no mathematical prerequisites.

The course was designed with the assistance of a Navy officer. The students practiced fuzzing and hacking (admittedly insecure) programs on old, virtual systems.  This was quite deliberate!  I didn't start by asking, "what will the kids be able to gain entry into at the end?"  Instead, I posed this question to the officer: "High school students are also quite young, so there is little pressure to give them useful skills right now. CTF competitions abound, and my students are already joining those. What is hard to pick up from CTFs, but nevertheless is ultimately quite central? What can we offer them that will help them develop into top minds in this field in the future?"  And that became the central mantra of the course.

Everything was done by hand at a *nix prompt, and they got a fair amount of practice reading and examining hexdumps.  By the end, they could look at a hexdump and see at a glance where, for instance, the text strings were stored.

It was ultimately a computer science course on binary and hexadecimal, on system variables, and on the intricate operations of a runtime stack.  As an additional bonus, they all got pretty adept at navigating a bash prompt.  The students kept asking for network hacking as well, though we didn't have time to get that far.  

The material was quite dense and unintuitive.  (Though, I suppose, if these practices were intuitive, they wouldn't have been vulnerabilities in the first place.)  It was an extremely difficult class, although the material itself kept the kids pretty interested, so you will have that working in your favor if you go down this road.