Getting ahead in the teaching game can be very difficult. You stay another year at your school, get paid a little more, maybe some more holiday, but that's about it. The longer you stay the more useless teachers you see come and go. You have that mixed feeling of knowing you're worth more in your school, but also how you are now the go-to-guy whenever there's a problem with people like '23-year-old James from Leeds' who loves getting leathered and indulging with the local ladies. He didn't go to uni, but he's wicked good at installing Sky satellite dishes. He thinks he got the job simply because he's special and handsome and finds it hard to cope with any criticism or requests to do work. The break from the honeymoon period has been hard on James. So bad he's thinking of getting a new job! He threatens this in his head everyday.
For every good teacher, I'd say there are three bad ones. The bad ones are usually allowed to coast along until the school finally get their revenge and don't renew their contract. Some like James from Leeds live from paycheck to paycheck so know they actually need their job. They'll come to work on time and not complain for the last few weeks. All the things anyone should be doing in their job anyway. This way the school might think James has turned a corner. At the very least for James, he has his "I've been treated unfairly" card for when the school wanna let him go. He can tell his friends and missus who quit her job when she met him how he arrived on time, taught his lessons and was loved by everyone at the school. Except for one person who had it in for him for 'no reason'. Life's so unfair for James.
Rinse and repeat in many schools. The good teachers watch the bad teachers come and go and wonder why nothing is changing. Often quoting Einstein's definition of insanity.