Let's briefly dip into the teacher's strike going on right now in Chicago. Charlie Pierce has the jig pretty well nailed down here in any case, but it's worth summarising and repeating: anyone who looks at a profession suffering combined funding and recruiting crises and figures the best thing to do is start reducing the work-force has replaced their brain with bubbling tar.
One of the biggest problems in education - and I admit upfront I only have anecdotal evidence to link my experiences to that of teachers in Illinois - is that the best teachers are often found at the best schools. This can hardly be considered surprising, of course, this kind of like-with-like clustering is true of almost any profession other than politics and whatever the fuck Mitt Romney tells himself he did before he spent seven years telling people he should be president because black people hate working for a living.
What this means in practice is the better the school, the easier it will find it to replace a teacher they don't think is bisecting the brassica seeds. Which inevitably makes it harder for the schools not having so good a time of it. Schools that would melt your face if you spent ten minutes in their playground. Schools where Cthulhu would cry into his tentacles if they asked him to cover a PE lesson.
These schools are often filled with wonderful pupils, and wonderful teachers, doing their best under circumstances that would crack most of us like a tea-cup between Megatron's thighs. This is a better recipe for producing Bolivian throat-warbling than it is high test scores.
So what happens when all those teachers get fired? Who do you persuade to replace the people who just got shit-canned for having miserable working conditions and stress levels that tend to be one student assault away from turning teachers into that poor fuck from Scanners? These places are already hurricanes of anguish and exhaustion and ingratitude and actual physical danger (a colleague of mine was once poisoned by a student who'd showed up early for a voluntary GCSE revision session), and the best idea these people can come up with is to decrease job security? You might as well try and deal with a beef shortage by banning goat meat.
Teaching is a tough job. Not everyone who wants to do it is going to be able to. It is not clear to me how we address this by making it less likely people are going to want to try.
P.S. Charlie is on the money with his observations about average wage, as well. It's always been a favourite tool of the right to utterly screw half the people over, then use their anger to justify screwing over the other half in the name of "fairness" (which, as Tomsk pointed out a while ago, is a concept which Cameron and Osborne have as much interest in as the ratio of gases in the Horsehead Nebula). And right now it's certainly not a trick limited to the the right-wingers in government over the pond, either.