A teacher cannot achieve tenure until they put in a minimum of three of the four years of effective or higher ratings and quite a few teachers need five years as the Principal or Superintendent may want to delay tenure by giving the teacher an extra year for a myriad of reasons. if a teacher gets a developing or ineffective rating for the fourth year, the Principal or Superintendent can discontinue the teacher. Usually, if a teacher needs a second year of the extension of probation, his or her days are probably numbered. I know of many New York City teachers who were discontinued when they received a second year of probation.
A teacher who was discontinued in one New York City district can theoretically get another position in a different New York City district or has a license in another subject but few have dual licenses . However, there is a problem. The DOE will tell the principal of that district not to hire the discontinued teacher and to ensure that happens the DOE puts the discontinued teacher on a "do not hire list" and places a problem code on their file. What Principal would go against the DOE? None that I know of.
Therefore, once a New York City teacher is discontinued, forget about getting a teaching job in the New York City public school system.