I'm not sure how the professor can claim that it's an application of the third law (ie f = ma) but there's no force involved; this would imply that there's no acceleration, which is clearly wrong.
There is acceleration. Therefore there is also a force, assuming that the rocket has mass.
Exactly where the force is applied is just a matter of design. The term "rocket" covers a lot of different designs and the force may not always be applied at the same point on each of them. As you say, in a conventional rocket the force is largely applied to the engine bell. In a simple compressed air rocket, force is applied pretty much everywhere except the exhaust opening.
Last edited by Slatye; 8th April 2013 at 10:58.