I am in NY.
The "At-Will" part of education law refers to teachers during the probationary status (tenure-track position). At the end of each year of the probationary term, the school can choose to discontinue the tenure-track probation. The school is allowed to grant tenure at any time they wish, generally it is after 3 years. If the district does not grant tenure after 3 years, tenure is automatically granted. Tenure is not a "job for life" as it is sometimes called. It basically provides due process for the tenured teacher.
The one comment I read on the link "The stupidintendent has a right and "WILL" fire you for anything he doesn't like, "insubordination", your personality, your clothes, your age, etc." has some errors. First, termination is not "for anything" , in fact no reason needs to be given. Second, terminating someone based on "age" is against federal law regardless, as are other protected classes. Although proving it would be difficult, if not impossible.
[url=http://public.leginfo.state.ny.us/LAWSSEAF.cgi?QUERYTYPE=LAWS+&QUERYDATA=$$EDN3012$$ @TXEDN03012+&LIST=SEA83+&BROWSER=33129109+&TOKEN=1 9589046+&TARGET=VIEW]NYSlaw[/url]