Here is a very nice site for teaching evolution from the University of California Museum of Paleontology. It is geared for K-12 science teachers and contains specific lesson plans for every grade level. Check out this comic explaining fitness.
I'm an evolutionary biologist who studies bacteriophage life history stochasticity and the population dynamics of host/pathogen interactions I'm currently affiliated with Queens College and the CUNY Graduate Center. I can be reached at john.dennehyATqc.cuny.edu