Let cellphones ring – just not in city public schools.

A state judge yesterday backed the city’s ban on cellphones in public schools, which was challenged by parents who argued the prohibition endangered their children in a post-9/11 world.

In rendering his opinion, Manhattan Supreme Court Justice Lewis Bart Stone noted that while parents suggested alternatives to the ban, “none of such suggestions are so compelling as to . . . find the cellphone rules faulty as a matter of law.”

He also noted that the rules “are not absolute,” and said principals could allow students to carry cellphones in school for use after class on school grounds.

Lawyers for the parents seized on that portion of the ruling, but remained disappointed overall with the decision.