Why are Friedel Crafts reaction not possible for molecules less activated than benzene?
Technically, the teacher's claim might be slightly modified.  Deactivated benzenoid aromatics do fail to undergo Friedel-Crafts reactions, but according to a summary description from the University of Calgary:

Deactivated benzenes are not reactive to Friedel-Crafts conditions, the benzene needs to be as or more reactive than a mono-halobenzene.

So, a monohalogenated ring which is only weakly deactivated, can still react.  However, stronger deactivation, for example with an acyl group, would kill the F-C reaction.
This property can actually be quite useful when we need to do the F-C reaction selectively.   Using acyl as the deactivating group, we can acylate an aromatic ring and be sure the reaction occurs only once.  The ring becomes too deactivated to react further.  In principle this can then lead to a monoalkylated product with good yield by subsequently reducing the acyl group, whereas direct alkykation which activates the ring could give polyalkylated products.