Not to be pithy, but I'd be more interested in the ability to fire bad parents.
Even a weak teacher can guide a class of students through the chapters of a textbook. Sure, the kids would learn a lot more from a good teacher, but they are shaped far more by their peers. Crummy parents produce rotten kids, who slow the class down, cause disruptions and set bad examples. Furthermore, there are many talented teachers who are poor baby sitters.
http://www.youtube.com/watch?v=u6XAPnuFjJc
It seems to me that the only piece missing from the current system is the ability to fire bad teachers.
I'm generally pro-union, but I believe unions do themselves a disservice by propping up bad teachers from legitimate dismissal as well as protecting good teachers from unfair treatment.