In NYS, if a Superintendent and the School Board are truly focused on student success and good teaching, it's relatively easy to weed out bad teachers. In many instances the teacher's union will "support" the decision if kept abreast of the process and the evidence produced by the process. However, the union's support will not be public, nor will it keep the union from publicly speaking against the firing.
The union will also seek some way to ease the separation such as a letter of reference. A district must never agree to such a demand. When contacted by an employer seeking info about the fired teacher, the only statement to be made is to say, "She/He worked in our district from date X to date Y." No matter the question, that same answer must be the only response. It usually only took two questions for the light bulb to go on at the other end of the telephone.