Over time it has become nearly impossible to fire a public school teacher once the teacher has tenure, a completely nonsensical situation created by the evolution of the profession of teaching into one nearly fully controlled by unionism.
This is ultimately tied to low-income and minority kids getting a much lower-quality education (because the worst teachers, when they can't be fired, are transferred into neighborhoods where parents have less power to do something about it). President Obama and Secretary Duncan are rightfully pushing on school leaders to change this.
One school superintendent made headlines for firing ALL the teachers at a long-time failing school. This may seem like an extreme strategy, but it's not: those teachers all get to reapply for their jobs and effective teachers will be rehired.