Mayor Bloomberg is getting upstate help in his bid to end the “last in, first out” law requiring that teacher layoffs be done by seniority instead of merit.

The heads of New York’s four other large urban school districts — Buffalo, Rochester, Syracuse and Yonkers — have co-signed a letter saying they want to abolish seniority-based layoffs in their cities as well.

“Our children deserve the finest educators, and some of the most innovative and effective teachers are not necessarily the most seasoned,” the letter said in part.

Buffalo state Assemblyman Sam Hoyt endorsed the campaign to repeal LIFO. He stressed any overhaul should cover all urban school districts, not just New York City.

Bloomberg has recommended laying off 4,660 teachers this fall because of an expected loss in state and federal aid.