E.D. Kain defends making teachers hard to fire. He argues that if "you take away pensions, job security, tenure, the ability to unionize, and basically all the other perks of teaching, what you’re left with is a very difficult job with no job security, mediocre benefits, and relatively low pay":
Charts such as this one are misleading for a number of reasons.
First, this chart only applies to tenured teachers. Bad teachers can be weeded out much quicker before gaining tenure. School officials need to use this time window appropriately.
Second, the point of tenure is to protect teachers from arbitrarily being fired. Teachers need protection from over-zealous bosses and ideological politicians. This is the same thinking behind seniority rules, which protect more expensive teachers (i.e. veterans) from being laid off due to budget cuts. Teaching is not a high-paying job compared to jobs in the private sector, and one of the benefits is some job security. Occasionally this means bad teachers take longer to fire.