A former school canteen manager, described by a judge as a lonely single mother, has avoided going to jail for having sex with a boy of 15.

The Adelaide District Court heard Kylie Fiona Henry was 38 when she slept with the teenager, who had stayed at her northern suburbs house with his younger sisters in 2011.

Henry originally was charged with rape but a jury convicted her of unlawful sexual intercourse.

Judge Paul Cuthbertson said the verdict suggested the boy either agreed to have sex or Henry failed to realise he did not agree.

He said Henry was a lonely single mother of three and in some respects she was someone to be pitied.

"Life has not dealt you a good hand of cards," he said.

"I have no doubt that you performed the voluntary work you did at the primary school to pass the time and to meet people.

"You were a friendly and outgoing adult to the children at the primary school."

Woman invited boy and his sisters to visit

The court heard Henry first met the boy when he was in primary school and years later he returned to the school to watch his younger sister's sports day.

Henry struck up a conversation with the boy's mother and he was invited to her house to do gardening and his sisters were invited over to play with Henry's children.

The boy and his sisters ended up sleeping over at the house.

The teenager kept watching television with Henry after the younger children went to bed and the pair later had sex.

"You were in the position of a parent, he was staying at your house with the permission of his mother," the judge said.

"[The victim and his mother] both suffered deeply as a result of your offending.

"You became obsessed with the complainant, visiting his home virtually every day for the next three months. Your computer communications via Facebook revealed your obsession."

The court heard jailing the woman might force her children into state care.

A two-year jail sentence was suspended and Henry was given a three-year good behaviour bond.