Up to half a million pupils are in schools without permanent head teachers, research by the National Association of Head Teachers suggests.
Heads want more time for planning, preparation and assessment
The union warns of a recruitment crisis in England, with the workload putting teachers off applying for promotion.
More than a quarter of school leaders surveyed would consider changing jobs if their heavy workload continued.
The Department for Education and Skills said "no government has done more to both recruit and support heads".
The union's research, involving 688 head teachers, deputy and assistant heads, revealed resentment about a perceived culture of excessive hours.
The union also surveyed a fifth of local authorities and found 257 schools without permanent heads.
If replicated across the country, this would mean up to half a million pupils in 1,200 schools without head teachers.
The union said reducing head teachers' work should be a government "priority".
A spokesman for the DfES said ministers "recognise the challenges the job brings".
The research suggested many felt they were not benefiting from the contractual changes implemented as a result of the National Workload Agreement.
It revealed that two-thirds of respondents taught but four out of five did not receive any time for planning, preparation and assessment.
And all respondents said they provided cover for teachers, with more than 20% covering between 41 and 100 hours in an academic year.
The findings revealed that over a third of head teacher respondents did not receive any "dedicated headship" time, while just over 15% of deputy and assistant head respondents received no leadership and management time.
The union said: "This survey further highlights unattractive aspects of the job which need to be remedied if the recruitment crisis is not to worsen."
Its general secretary, Mick Brookes, said: "The issue of recruitment and retention to school leadership is crucial to the success of the education system.
"The government must make this a number one priority."
A DfES spokesman said: "It is important to keep this in context - vacancy rates for head teachers have fallen significantly with only 0.7% of posts now vacant compared to 1% in 1998.
"But we are supporting heads further, including record pay, slashed the bureaucratic burden on them, key reforms to help them manage workloads better and changes to promote leadership so they have the skills needed.
"Since 1997 the maximum head teachers can be paid has risen by more than 35% in real terms from £56,676 to £93,297, with over £102,000 available in inner London from this September."
He added: "Today's workforce statistics show there are more teachers and support staff in our schools than at anytime since 1981, up 36,200 since 1997, thanks to our commitments on pay and conditions."
The research was carried out by the Centre for Industrial Relations, Keele University, in conjunction with the Work Life Balance Centre in Leicester, for the NAHT.
The internet-based survey was conducted between September and November 2005.