Is it unlawful for a Kohen to go to medical school, since they study on cadavers? And doctors pronounce people dead? Since they are not to defile themselves with the dead it seems to be wrong.

Raybin,

Technically, yes, Kohanim could not go to a traditional medical school. However, things have been changing. One can study anatomy in med school today without using cadavres and one can go into specialties that do not require encounter with deathly ill patients. Residency is tricky but it can be done.

The bigger question is that a kohen cannot be under the same roof with a dead body. What does that mean...the same room, building, facility or what? That has been the bigger debate.