anything with easily absorbed sugars and starches (as YT said)
not being a dietrician myself, I would suggest spaghetti and peanut butter (just because I think it would be funny if someone actually ate that)
I think the only thing that messes me up in math tests is when I try to actually show the work, I CAN'T DO IT!!! I hate when teachers write "show your work" because then I'll try, and I'll get something messed up:-p
however, rarely do I ever eat breakfast, and I was one of the smarter people (maybe) in my high school, so it doesn't really bother me....but glider is a lot righter (grammar for ya sayo) than I am, so listen to him