One quarter of secondary schools in Lancashire are falling behind the required standard, the education watchdog says.
Of the 83 schools in the area, Ofsted rates seven as inadequate, its lowest mark, while 14 require improvement, as of September 30. Its latest figures list 18 as outstanding and 44 as good.
The regulator visits all new schools, including academies, within three years of opening.
Inspectors judge them on categories including the quality of teaching, personal development and welfare, the effectiveness of the leadership and pupils' achievement.
Schools requiring improvement will be inspected again within 30 months, while those rated inadequate now face mandatory conversion into academies, funded directly by central government.
In Lancashire, there are 630 schools registered with Ofsted including primaries, 13 of which are rated inadequate while 47 require improvement – meaning 10 per cent overall are below standard.
This is, though, slightly below the 13 per cent average for the North West.
Across England, 20 per cent of all schools were classed as outstanding, 66 per cent good, 10 per cent requires improvement and four per cent inadequate.
But with more than 1,000 outstanding state schools going without an inspection in a decade, the National Education Union warned this did not accurately reflect the quality of education they offer.
Dr Mary Bousted, the union's joint general secretary, said: "The fact that some schools haven't been inspected for over 10 years demonstrates that the information Ofsted provides is misleading at best and may be downright wrong. This is yet another reason that Ofsted is past its sell-by date."
The Department for Education recently announced it will consult on plans to remove the exemption for outstanding schools, a move Ofsted says it welcomes.
"This is something that Ofsted has long argued for, so we're pleased the Government has made this announcement," said a spokeswoman for the regulator.
"Routine inspection assures both parents and schools that the quality of education on offer is of a good standard. In the interim, inspectors can and do go into outstanding rated schools if necessary.
"We have powers to inspect at any time if we have concerns about the standard of education, or if there is a safeguarding concern."
A DfE spokeswoman added: "This Government is committed to providing world-class education for all students and, where a school is judged as inadequate by Ofsted, the Department will not hesitate to step in and ensure that swift improvements are made so that all children at the school can receive the education they deserve.
"The gap between disadvantaged pupils and their peers has narrowed considerably in both primary and secondary schools since 2011.
"Teachers and school leaders are helping to drive up standards right across the country, with 85% of children now in good or outstanding schools compared to just 66 per cent in 2010."