The city's only primary school in special measures is making inadequate progress, according to its first Ofsted monitoring report.
Inspectors gave Paulsgrove Primary the worst possible grade for three out of four categories including pupils' achievement, quality of teaching and learning, leadership and management.
This report comes two months after Isambard Brunel Juniors and Meredith Infants, both rated satisfactory at their last full Ofsted inspection, were slammed with inadequate monitoring reports.
In all three schools, Portsmouth City Council has been providing support through school improvement plans.
Cllr Alistair Thompson, the Conservative spokesman for education in the city, said: 'The responsibility and power rests with schools and governors, but clearly the local education authority has an important supportive and monitoring role and it is not doing the job properly.
'We've been told in the past that the system is robust and we can pick up problems. But the proof of the pudding is in the eating - generations of children have been failed.'
In September, Andrew Olive, previously in charge of learning and achievement at the city council, was appointed interim head Paulsgrove Primary.
Four months earlier, Ofsted gave the lowest grades in 24 out of 28 areas. But this latest monitoring report says progress since then was inadequate.
It reads: 'Pupils' attainment at the end of year six remains exceptionally low. Too little emphasis is placed on using assessment consistently, leading to the continuing under-performance of a significant number of pupils.
'There are too many tasks set which are of little value in enabling pupils to learn.
'A number of staff do not yet view themselves as leaders in the classroom and this presents a barrier to improving the inadequate teaching.'
Mike Fowler, the council's head of transforming education services, pointed out the monitoring report's approval of measures put in place. He said: 'Ofsted recognised we were doing all the right things, but as Andrew and his team started just seven weeks prior to the visit, it will take longer to see the full benefits. The case of Meredith Infants and Isambard Juniors is different because they were in the satisfactory Ofsted category.'
He added: 'This council has a good record when it comes to supporting schools in a category.
'We got Cottage Grove out well under the expected time, and we have just seen Somers Park Primary come out of notice to improve.'