A judge has acquitted a Toronto teacher of sexually assaulting her former student when he was 15, but not before he chastised her for getting so deeply involved.

Mary Gowans “showed poor professional judgment,” Ontario Superior Court Justice John McMahon said in delivering his ruling Friday.

He cited the veteran teacher’s frequent use of the student as a babysitter after he graduated from her elementary school and her allowing him to help with after-class athletic activities.

“I would have thought that her 17 years of teaching experience, she would have realized . . . he’s spending a lot of time in her life,” the judge said.

“Alarm bells should have gone off.”

He acquitted Gowans, 43, who has resigned from the Toronto public board, of sexually assaulting and sexually interfering with the youth when he was 14 and 15.

The dramatic two-week trial saw the youth, now a tall, athletic 18-year-old, take the stand to paint a lurid tale of how she groomed him into having a sexual relationship, involving heavy petting, in late 2009 and January 2010.

The complainant “may have been telling the truth” but his evidence was inconsistent, both internally and with some of the facts, McMahon said

“It is unsafe in law to base a conviction on his evidence,” the judge said.

After the verdict, Gowans wept and hugged her lawyer, Peter Brauti, and family members.

Outside court, Brauti said his client was “incredibly relieved this case is over.”

However, Brauti noted, “the damage that it’s done to her, her family, her reputation, regardless of the fact she was acquitted today . . . will be difficult to recover from, if ever.”

Gowans did not answer whether she plans to return to teaching, but Brauti said he doubts she will. “The whole thing has been such a negative experience,” he said.

Her voice trembling, Gowans told reporters, “It has been a very, very difficult week.

“The last three years of my life have been virtually destroyed as a result of these allegations,” she said.

“I’m glad it’s over, but at the same time I have the task of rebuilding my life now.”

She thanked her supporters, including her family and friends. “Just let everybody know I didn’t do anything wrong,” she added.

The student refused to answer the Star’s questions as he walked along a court hallway, but his mother said the family still believes he is telling the truth.

“I believe she was lying,” the mother said of the teacher.

“We’re disappointed with the verdict but we believe our son, and Ms. Gowans can live with her lies.”

While McMahon found the teacher to be largely credible, he said he could not accept her claim she had no idea the boy was sexually obsessed with her.

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McMahon said he was troubled by the more than 2,000 text messages the pair exchanged, and the fact that the teen was the first person she texted both Christmas morning and New Year’s Day.

She explained that many of the messages, all of which have been erased, had to do with babysitting and their volunteer athletic activities.

The judge also wondered how the youth knew she had breast enhancements and a groomed pubic area, but found this knowledge not to be determinative.

The teen is exceptionally bright and confident, but he was sometimes confrontational and argumentative on the stand, McMahon said.

When the youth first testified, McMahon said, the judge believed he just had a schoolboy crush. But in cross-examination, the teen’s behavior and comments seemed more troublesome, the judge added.

It came out the boy frequently discussed this “hot” teacher with classmates, saying he liked to see her breasts bounce when she jogged, and he wanted to have sex with her.

He could give few details in court about the first time he claims he touched her breast although he admitted he had long dreamed of doing so, the judge said. “It defies common sense,” he added.

The judge also had trouble believing the youth’s claim she showed him her breasts through a wet T-shirt in June 2009 as her payment on a bet.

The teenager claimed she locked her two children, 5 and 9, out of the house while they were in her backyard pool so she could show the youth her breasts. “It just does not ring true,” the judge said.

But McMahon found the youth’s accounts of his heavy petting sessions with the teacher between November 2009 and Jan. 5, 2010, detailed and generally consistent.

He found inconsistencies, however, in the teen’s explanations of why he confessed the alleged affair to his sister, calling them “somewhat troubling.”

One reason for confessing, the teen said, was he felt overwhelmed after his teacher had invited him to spend several hours alone with her while her husband was away.

“It seems odd that the complainant, who was telling his friends he would have sex with the teacher, was not willing to take the next step,” the judge wrote.

The judge said the student’s other reason for confessing to his sister — that during a 40-minute car ride the teacher was tearfully pressuring him to stop dating a girl his own age — does not accord with other facts and clashes with his testimony at the preliminary hearing.