Bryan Caplan hits the nail on the head:
I have a simple solution: stretch the scale upwards. If students call 60% of their professors "excellent," we need to add stronger adjectives to the list of responses. I suggest we add 6="best professor I’ve had this year" and 7="best professor I’ve ever had."
I still suspect students would overuse these options – during their four years, a student might give out ten 6’s and five 7’s, instead of four 6’s and one 7 like they should. But my reform would publicly distinguish teachers who do their job and appease complainers from professors who change their students’ lives but refuse to coddle them.