Harry's scar and Quirrell-mort

[irenetsui]

No: HPFGUIDX 164614

I had been thinking about this for many days now and I wonder if this
question has been brought up before...

During Harry's first year at Hogwarts, when Voldemort was sharing
Quirrell's body, hiding at the back of his head under the turban, how
come we did not hear about Harry's scar hurting in DADA classes? We
know his scar hurt at the Welcoming Feast when Quirrell had his head
facing Harry. I am sure Quirrell would have turned his back to the
students at least once in a while?

Thanks! 

Irene