Ok, I'm not sure if this is even related to the problem, but just in case, here's the overhead view of the Hall of the Elements. The greenish box is the lesson area, which I understand to function as a trigger for the NPCs to begin their 'argument' about practicality vs safety, when the player crosses into it.
The red directional box on the right seal is the tolfdir casting marker, and the left (facing it) is the student casting marker. Try centering yourself about where that box is, and see if it triggers him.
If this is a bug, and not just a matter of being slightly off a line, then this won't help, and I'm not sure what will.
You can try to reset his AI, if you wish, by opening console with ~ and pointing at Tolfdir while you're in console. Click on him and you'll see his number come up, under your crosshairs. Type
resetai
and see if that helps. You could also try
resurrect
or
recycleactor
This is the page on the console commands, which will describe them and how to use them. ALWAYS save first.
http://www.uesp.net/wiki/Skyrim:Console
Here's the UESP on the quest. Down in Notes and Bugs, it describes a couple of options you may try. If you don't want to kill a student, you may opt to try making a save, killing Tolfdir, and then immediately reloading the save. I don't know if that will reset him or not, but it works for merchants, to change what they have in stock.
http://www.uesp.net/wiki/Skyrim:First_Lessons
Hopefully something in here will help you.