School change obscures performance, says heads' leader
Continuous change has made it impossible to tell how well England's schools are performing, said a head teachers' leader.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), was addressing its annual conference in London.
Students' exam grades at 16 "cannot be compared from year to year", he told the union's delegates.
The government said its reforms had been "necessary and urgent".
Mr Lightman called for a period of "clarity and stability" after rapid reform.
At the moment, understanding how well the system is doing "is a real challenge", said Mr Lightman.
"Hard though it is to believe, I am standing here and stating that we do not know how well our system is performing in terms of attainment in our qualifications for 16-year-olds and may not for some years to come," he added.
Mr Lightman promised that ASCL will continue to work with awarding bodies and officials "to get to a stage very rapidly when teachers, students, parents, employers and the government know exactly what each grade means in terms of learning outcomes and how standards compare between years".
The union is calling for schools to have a greater say in setting the school curriculum and for the government to put more trust in teachers rather than imposing "relentless change from above".
In particular the trade union wants the national curriculum to be decided by an independent commission made up of school leaders, governors, teachers, parents, employers and politicians.
It wants the curriculum to be reviewed only once every five years.
A Department for Education spokeswoman said reform had been needed as children "only get one chance at education".
"We know we have expected a lot from teachers and appreciate the huge efforts that head teachers and schools have made to implement these reforms.
"Together we are transforming education with one million more children now being taught in good or outstanding schools than in 2010.
"The important thing now is to build on this success and maintain a period of stability to allow the reforms to bed in and standards in our schools to continue to improve."