Parents and teachers said they believed the proposed closure encouraged families to move their children before councillors had reached a final decision.
The council acknowledged some families had moved early but said parents were entitled to seek places elsewhere at any time.
A spokesperson said the council could not guarantee a child a place at a particular school, because admissions decisions depended on each school’s arrangements, but added that three primary schools within 1 mile (1.6km) of St Teresa’s had vacancies, along with two Catholic schools within 1.5 miles (2.4km).
Council figures show more than 50 pupils secured places at other primary schools before the decision, leaving just 39 on its roll.
Helen Fitzpatrick, whose daughter goes to the school, said families were “absolutely devastated” by the decision, announced on Monday.
The 39-year-old, who has helped organise a campaign against the closure, said she had contacted nearby St Ann’s RC Primary School but was told it could not guarantee a place for September 2027.
Although a place was available when she asked, she said she would have had to accept it immediately and her daughter was not “ready to leave St Teresa’s until it closes”.
She said she also feared her child could “get lost in the crowd” at a larger school.