A Bucks County middle school teacher has been charged with sexually assaulting two of his ninth-grade students after propositioning the girls over the Internet and on their cell phones.

Jeffrey Anderson, 23, of Northeast Philadelphia, was arrested Monday night and charged with crimes including involuntary deviate sexual intercourse, aggravated indecent assault, and criminal use of communications devices.

Anderson had been the building substitute at Tohickon Middle School in Plumstead Township. Detectives pulled him out of his classroom Monday for questioning, and he provided a 14-page confession, court records say.

Anderson was fired immediately by the Central Bucks School District, where the La Salle University graduate had worked since 2005.

According to court records, Anderson admitted he kissed, fondled and digitally penetrated the girls, ages 15 and 14, inside the 15-year-old's house on a recent Saturday afternoon. He also engaged in oral sex with the younger girl during the April 28 meeting, the records say.

If convicted of the most serious crimes, Anderson faces a mandatory minimum five years in state prison. He is being held in Bucks County prison in lieu of $500,000 bail.

The girls' parents had been alerted by computer monitoring software that their daughters had been communicating with the teacher via instant messaging and Facebook.com personal Web sites, District Attorney Diane Gibbons said. They confronted the girls, who admitted engaging in sexual acts with Anderson, a probable-cause affidavit said.

Both girls had been in Anderson's class, and had been messaging him outside of school for about a month. In some exchanges, Anderson told the girls he wanted to have sex with them, the affidavit says, and recently met with them in a Tohickon classroom and kissed them.

In an April 22 Facebook posting, Anderson pleaded with the girls to keep things quiet, the affidavit says.

"PLEASE . . . nothing that is said or done between the three of us can EVER be repeated to anyone else!!!" the message said. "Not your best friend, your diary, or any of those girls at school!! I could get fired, never become a teacher again. or go to jail. . ."

Five days later, after an explicit proposition to the girls on Facebook, Anderson allegedly added: "Hah . . . im so going to hell . . . I can't believe I hooked up with two 9th graders!!"

On Saturday, April 28, the affidavit says, Anderson was text-messaging the girls on his cell phone when he learned the 15-year-old's parents would be away for the day. At about 4 p.m., he parked his vehicle down the block from the girl's house and met both girls inside, where they engaged in sexual acts.

A few days later, Anderson met the 14-year-old inside a Tohickon chorus room and kissed her, the affidavit said.

The parents of the girls had installed on the girls' computers software called IamBigBrother. It gives parents the ability to capture and monitor incoming and outgoing e-mail, as well as Web sites visited, passwords entered, and basically everything typed, both online and offline. Gibbons said that the girls knew their parents were using this software.

The charges echo a 2003 case in which Ryan Newman, a popular young art teacher at neighboring Holicong Middle School, went to prison for at least five years after admitting to an affair with a 15-year-old student.

In that case, too, the victim's parents were alerted by reading an instant-message thread their daughter had left on her computer. Newman was well-known among students for chatting online, and some even stopped by his apartment in Doylestown borough to socialize.

Like many districts, Central Bucks has a general policy prohibiting offensive contact or relationships between students and employees, but does not specifically ban fraternization outside of school.

In the case of predators, Gibbons said, a tougher policy might not matter. "I don't think it would stop them," she said.

Contact staff writer Larry King at 215-345-0446 or lking@phillynews.com.