Seek, this is yet another of the things widely known which happen not to be true. Believe it was Mark Twain who said "It ain't the things that we don't know that get us into trouble, it's the things we know for sure that ain't true".
As Chris and others have pointed out, the lower impedance hinders the input voltage less and the result is that for a given voltage the current is higher. This is because of Ohm's Law, which I'm sure that your instructor is familiar with, even he's not much into audio technology. One form is I=E/R(current equals voltage divided by resistance[impedance for AC]), which shows the effect of the lower impedance. Since another iteration of Ohm's Law is that power equals voltage times current, the effect of the higher current is also higher power for a given input voltage from the amplifier.
So, in that sense lower impedance makes it easier to realize power in the speaker, but the higher current possibly could cause overheating in the amplifier and its protective circuits would shut it down. However, there's no brick wall suddenly created by impedance lower than some magic number, as some seem to imagine. It's entirely possible for some speakers rated at 4 ohms(e.g. the M80)to be more sensitive than some speakers rated at 8 ohms, to require less power for a given sound level, and to be less likely to cause amplifier shutdown.
Let us know what your instructor thinks about this.
