The problem comes from judging good teachers from bad; we simply don't have a good way to do this. You absolutely CAN NOT use the performance of the teacher's students, because the students themselves have too large an effect. Also standardised exams vaguely fail at assessing good students anyway; they're a necessary evil for post-secondary entrance and all that, but tieing any more than is absolutely necessary to them strikes me as a Very Bad Thing.
I am, of course, very biased on the matter, but one of the biggest things that helps is not underpaying your teachers to hell and back. It's not tooo big a problem where I live, but god knows it is in certain parts of the US (under 30k for an annual salary? What the fuck). Sadly, if you pay people that little, you send a message that their job isn't valuable, so you have more trouble attracting people, and give schools less to choose from. Note that finding quality doctors is rarely a problem, because it's a highly-paid, highly-honoured profession which a lot of people would like to be, so medical schools can be very picky about who they enroll and who they pass, etc.
In my education program, I can already see people I think will be outstanding teachers... and people I think will be quite the opposite, if they even make it (and the numbers say some of them will). Unfortunately I have little faith in any system sieving one from the other. "Good teaching" is easy to see but hard to prove. Unless you give schools the ability to let go teachers whenever they want without hard justification, which (a) isn't something we really tolerate in any workplace, let alone a unionised one, and (b) relies on the administration being both competent and uncorrupt, I'm just not sure what you can do.
As for the summer break issue... I dunno. The break is arguably a bit too long, though the Completely Biased part of me sure as heck doesn't want it getting shorter. I think true year-round schooling just fails, though - a large block of time off is necessary for students who want to take vacations, e.g. to visit family in other continents, because you really do not want these kids leaving during regular classes. Other than that, I will disagree with Dune slightly - usually, by the end of those months, I was glad to be back. Yes, they were awesome, but at a certain point it can be too much, and you just start missing that time with your friends, not to mention the direction in your life.
Granted, I tend to think the school year is long enough as is, so any shortening that happened to summer I'd be inclined to replace with days or weeks off in the middle of the year.