NY- A Manhattan high school administrator who bragged about sexual liaisons with teenagers was nabbed trying to hook up with more underage girls in an Internet sting operation, authorities said yesterday.

Steven Mark Rubinstein, 49, an assistant principal at Murry Bergtraum High School, pleaded guilty to kiddie porn and child enticement charges in White Plains Federal Court.

School officials reassigned Rubinstein last month after learning of the allegations and will move to fire him, a Department of Education spokeswoman said.

Using the handle “DaddyMark,” Rubinstein sent sexually explicit messages from his home and school computers to federal agents posing as 13-year-olds over the past year, prosecutors said.

Rubinstein, who lives in Poughkeepsie, claimed in the cyberchats that he’d had sex with 15-year-old and 17-year-old girls in the past.

After trading messages with the undercover officers, he requested a meeting so he could add them to his list of conquests.

When confronted, Rubinstein admitted trying to meet the “girls” but shied away from an actual rendezvous when he became suspicious that he was dealing with officers.

A teacher since 1985, Rubinstein also admitted downloading and trading images of child pornography over the Internet. Authorities said they seized images of minors having sex with adults from his computer.

Rubinstein faces a mandatory minimum of five years in prison when he is sentenced July 10.