maybe there is a script issue? try loading from an earlier save or the autosave from when you enter the hall of elements. and are you exactly in the circle?
I've found he has a habit of just standing there when I cast my ward and then when I'm out of magic and my ward goes down, he tells me to cast it. I'm all like "Give me a moment!"
I've had this happen before and either someone else was close enough to Tolfdir's line of fire so that he doesn't fire OR you aren't in the sweet spot of that circle you are supposed to stand in.
Just step right up as close as you can to the NPC's face and stand there. The NPC will step back in response. Then get in his face again to force him to step back some more. Repeat until he leaves the space you want vacated.
It's literally taken me a few minutes of shuffling around and then casting the ward before Tolfdir finally shoots the spell. Annoying.
Do you have a auto-save from entering the Hall of Elements? Otherwise maybe just shuffle around the circle until he reacts. Tolfdir is a finicky fellow. I'll stand in the circle and cast, and he'll just stare at me. Then when I'm out of magicka, he'll tell me to cast a ward.
Maybe hotkeying a magika potion and chugging it when you run out will get him to cast.
Run over to the other side of the room and tell Lydia to wait then back to Tolfdir
Nice.
I wonder if Tolfdir and those students were standing around all that time just waiting on you...