You're not seriously doubting that public unions in general, and especially teachers unions, make it extremely difficult to get rid of poor performers, are you? I worked for the government for many years and saw first hand how poor performers were just shunted aside with lower levels of responsibility, because that was much easier than trying to fight the unions and fire them. Teachers unions also don't like students being given a choice of going to private or charter schools.
The state of Lousiana has instituted educational reforms including a voucher program that would allow students to attend schools of their choice, but lawyers for the teacher's union there are threatening to sue private schools that accept vouchers. Bullying pure and simple. The union lawyers know they don't have much chance of winning in court, but that's not the point. Many private schools probably don't have the money for lengthy court proceedings, so the union lawyers are trying to intimidate them.
http://blog.heritage.org/2012/07/27...ative+Policy+News.)&utm_content=Google+Reader