Mary denied this.
She claimed that last month, Sally's husband took a photograph of her. She also alleged that he told her that he would get her and her daughter out of the school.
When contacted, Sally's husband did not want to comment on the allegations.
Sally, however, said her husband once apologised to Mary about the disagreements.
The principal told The New Paper: "The school subsequently engaged both of them to settle the matter amicably."
Both mothers were given different times to enter the school to avoid further hostility.
Sally was allowed in from 1.30pm to 2.30pm, and Mary from 2.30pm to 3.30pm.
Letters were issued to them about this.
The mothers were told that if they did not adhere to their given times, they would be reminded to do so by school security officers.
The New Paper understands that Sally kept to the times.
But Mary has refused to leave the school on several occasions,and would sometimes stay as late as 6pm.
The principal told The New Paper: "Despite constant reminders by the security officers however, (Mary) persistently overstayed beyond the time given to her.
"Hence, the school had to seek the assistance of the police on April 6 and 7 when she continued to overstay."
The principal added: "The school will continue to engage both parents and seek their cooperation in maintaining order in the school."
But the problem between the two women has not been confined to the school walls.
Sally's lives in a block next to the school and after school, she sometimes takes her daughter to the void deck.
Sally insisted that she needed a place to watch her daughter eat, and that the void deck was more suitable than her flat because it was "more cooling there than at home".
But Mary claimed that Sally stayed in the void deck to"ambush" her and take her photo.
Mary said: "I was afraid that if I left the school, (Sally) would take my photograph from her block's void deck".
She said that was the reason she "overstayed" in school.
But Sally denied ever intending to take Mary's picture, adding: "Why should I take her picture? And why should she be afraid of people taking her picture?
"I can't help it if (Mary) doesn't want to leave the school."
But Mary said she is still suspicious.
So why doesn't she leave the school through another gate to avoid Sally?
"I can't do that for the rest of my life", Mary said.
"Using another gate would just be a short-term solution."
This article was first published in The New Paper.