It isn't totally unrealistic. I rewatched Red Dawn a few weeks ago and it has a different impact today and more resembles Iraqi insurgents vs. Coalition troops.
Or an even better example is Russia vs. Chechen rebels. Or Russia in Afghanistan. In all cases under armed and under trained young men and women are royally fucking up a huge invading army's shit.
I remember first seeing Red Dawn as a kid and thinking it was bogus. We were all lead to believe if WW3 happened there would be nothing left to invade after the first day. Now things have changed. Our army is way smaller, and too reliant on technology. Everybody hates the US, and the few allies left will switch if the opportunity arises.