AFIK you cannot upgrade any of the Student/Teacher editions to the next release. The rules that I am familiar with state that if you qualify for a Student/Teacher edition you can buy only 1 copy a year and then you have to buy the next version at whatever price you qualify for. There is no upgrade path.
I don't know what the right answer is but Adobe just told me the exact opposite. I also have a student version of CS5 (for PC) and was looking to swap it for Mac. They said I can only do that with a current version but that I could upgrade to CS6 instead while doing a cross-plat form swap. The weird thing is that they said I could keep using CS5 on my PC.
They seem to have some weird policies and it depends on who you talk to. The difference between a new student version and the upgrade is about 50 bucks.