It's more than 3 years now since a very close friend died of cancer. At the time, I wrote briefly of how cancer cell lines can evolve resistance to chemotherapy. Now Orac has written a much longer essay discussing the same thing. It's well worth reading & would probably make an excellent resource for working with senior school biology students.
Orac ends his essay with the following quote, an answer to those who ask why we have yet to cure cancer (even when using personalised therapies that in some cases target the genes themselves):
The reason we haven't cured cancer yet is because we haven't figured out how to overcome the power of evolution. Right now, cancer seems almost always to find a way. Until we figure out a way how to block all the ways it can find, personalised therapy will be effective in only a small proportion of cases.