By Angela Harrison BBC News, at the ASCL conference

Not enough teachers want to become heads, the union says

Head teachers are increasingly vulnerable and some lose jobs because their faces do not fit, school leaders have suggested.

School and college heads meeting in London this weekend were told vulnerability, responsibility and accountability had all increased.

The Association of School and College Leaders said 76 heads and deputies in the union were forced out last year.

Many heads lost their jobs before or after Ofsted visits, it said.

Of the 76 who were forced to resign last year, 28 were school heads in the state sector, 26 were deputy or assistant heads in the state sector while 22 were in independent schools or in colleges.

Nervous authorities

John Dunford, general secretary of the ASCL, told delegates the figures for 2006 were an increase and the trend for 2007 was even higher.

"Too often ASCL members are only one poor inspection grade away from their P45," he said.

"Too often, ASCL members are sacked precipitately before an Ofsted inspection by nervous local authorities that have previously failed to support the school in its difficulties.

"Sometimes the dismissal results from DfES pressure on the local authority or governors. Others result from a school becoming an academy and the leader's face not fitting."

It is your hard work which has led to today's students achieving better results Alan Johnson, Education Secretary

The union says future supplies of head teachers and college leaders are uncertain because there are insufficient incentives for teachers to take on the rewarding but demanding role.

It says there is not enough of a pay gap between the highest paid classroom teachers and those in management.

The government says the proportion of empty head teacher posts is less than 1% and is "stable".

At the conference on Friday, Education Secretary Alan Johnson had won approval from many of the head teachers.

He had praised them, saying: "It is your hard work which has led to today's students achieving better results, at every stage, in every part of the country.