Yeah, I have to agree with Aekiel here - just because Hogwarts professor's never mentioned it doesn't mean it doesn't exist. It's not like teachers explain to you all the dreadful things that can happen in the world... or if you want a few Harry Potter examples: Slughorn told Tom Riddle about horcruxes and then look what happened. If Snape was having his sixth year Slytherin students brew potions that could bring tissue back to life after being previously dead, someone might catch on. If Moody showed the class a charm capable of making a cadaver receive commands someone might practice outside of class.

I think necromancy is possible, even propable, especially for a young, semi-trained Harry Potter that doesn't really know what he's getting into but would just love to see his parents one time. Oops! I didn't realize that my resurrected mother would eat your brains, Hermione! Boy, she looks hungry too...