A teacher faces up to 40 years in jail for exposing her pupils to online pornography, amid an outcry from computer experts that she is the innocent victim of malicious software.

In a case that has become a cause célébre in the online world, where millions of rogue websites appear unsolicited on computer screens every day, Julie Amero is gathering a network of supporters who claim that she has been wrongly convicted over an incident she says has destroyed her life.

Amero, a supply teacher in the small Connecticut town of Windham, was convicted last month for exposing her class of 12-year-olds to graphic sexual images on the classroom computer. She contends the images were inadvertently thrust onto the screen by malicious software that she was powerless to stop. “I’m scared,” said Amero, 40. “I’m just beside myself over something I didn’t do.”

The case has become the internet-age equivalent of a teacher accused by a pupil of sexual assault, but this time — as one blogger in support of Amero claims — it appears that she has been “framed by the computer”. Many computer users say that what has befallen her could happen to anybody.

In October 2004 Amero was assigned to a seventh-grade class at Kelly Middle School in Norwich, a city of about 37,000. The regular teacher had logged on that morning. Amero says that before the class started, she sent a quick e-mail to her husband, and then went to the lavatory. She returned to find the permanent teacher gone and two students viewing a hairstyle site.

Shortly afterwards, she says, pornographic advertisements flooded the screen. She says she tried to click them off, but they kept popping up, and the barrage lasted all day. She tried to stop the students looking at the screen, but several saw sexually explicit photographs. It was school policy not to turn off computers.

Two days later she was suspended and was then arrested and charged with risk of causing injury to a minor. She rejected a plea-bargain deal that would have kept her out of jail. At her three-day trial, prosecutors argued that Amero was actively searching the web for pornography during the class.

Prosecutors relied heavily on testimony from a computer crimes police officer, Mark Lounsbury, who admitted that the software used to analyse the computer could not distinguish between mouse clicks and automatic redirects caused by malicious software. Herb Horner, a defence witness and computer expert, said that the children had visited an innocent hairstyle website and were then redirected to another site with pornographic links. “It can happen to anybody,” Mr Horner said.

Crucially to Amero’s case, the school has admitted that the computer had no firewall because it had not paid the bill.

Amero has since discovered that Mr Lounsbury never searched to see if the computer had become infected. “That is a blunder akin to not checking for fingerprints at the crime scene,” Alex Eckelberry, the president of a Florida software company, said. “When a pop-up occurs on a computer, it will get shown as a visited website.”

But Mark Steinmetz, who served on the jury, insists that Amero is guilty. “I would not want my child in her class. All she had to do was throw a coat over it or unplug it.” She says she panicked and did not know how to switch off the computer.

Scott Fain, the school principal, said that Amero was the only teacher to report a problem with the computer. “We’ve never had a problem with pop-ups before or since.”

Sentencing was adjourned yesterday until March 29. The maximum sentence is 40 years, although lawyers suggest that an 18-month jail term is possible. Amero and her husband have opened a blog, julieamer. blogspot.com, asking for contributions to her defence fund so that she can appeal.

She wrote: “One day you have the world on a string and the next day the string is cut and you are left falling into an abyss of legal, ethical and social upheaval. Why am I being persecuted for something I had no control over?”

Explore Tech & Web

• Personal Tech

• The Web

• Gadgets & Gaming