There are too many lacklustre schools in England which are not pushing children to reach their potential, says the annual report from Ofsted.
The education watchdog's yearly report says too many schools are failing to rise above the "satisfactory" grade.
There are also concerns about teaching quality, which was no better than satisfactory in 41% of schools.
The report also warned that schools serving the poorest communities were more likely to be rated as inadequate.
Miriam Rosen, the acting chief inspector of schools, said: "It is of great concern to see high numbers of schools, colleges and childcare providers that are consistently delivering services for children and young people that are no better than satisfactory.
"Ensuring that there is real improvement must be a matter of urgency for these organisations."
The education watchdog's report is the latest warning about "coasting" schools, often in prosperous areas, where schools might achieve respectable results, but fail to stretch pupils.
The report says that 800 schools have been judged as "satisfactory" in two successive inspections.
This "satisfactory" grade is above the point at which intervention is required, but below the higher grades of good and outstanding.