saycheese - one poster has already said the important thing, the explanation you seek refers to the equal transit theory which is not held in high regard because it leads to the very question you ask which is not answerable in those terms. (ie the overall explanation is really wrong/poor).
Google the coanda effect for more info. My understanding is that lift is caused by a combination of the air flowing over the upper surface sticking to it (coanda effect) and thus following the curve downward at the back (and thus being accelerated downward) and also the air striking the lower surface of the airfoil being accelerated downward by virtue of its shape.
The two accererations/deflections produce a reaction (newton) on the airfoil in an upward direction.
Stalls occur when the air on the top unsticks.
I expect someone who is better informed/has the correct instructors explanation can put some more detail to this...