A third of secondary schools in the East Riding are falling behind the required standard, the education watchdog says.
Of the 18 schools in the area, Ofsted rates two as inadequate, its lowest mark, while four require improvement, as of 30 September.
Its latest figures list one as outstanding and 11 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 the East Riding, there are 149 schools registered with Ofsted including primaries, seven of which are rated inadequate while 19 require improvement – meaning 17% overall are below standard.
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.”
The Department for Education (DfE) recently announced it will consult on plans to remove the exemption for outstanding schools, a move Ofsted says it welcomes.
A DfE spokeswoman said: “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.”