By Angela Harrison
BBC News education correspondent, Liverpool
Teachers are concerned about a two tier system of "technical schools"
Teenagers in England are being forced to make crucial decisions about their futures at too young an age, teachers have claimed.
Members of the National Union of Teachers warned of a return to a "two-tier" secondary system.
They attacked Conservative plans for technical schools and voted to campaign against any selection at the age of 14.
Conservative schools spokesman Michael Gove said the proposed technical schools would be "prestigious".
The Conservatives want to open 12 technical schools linked to universities and industries in England's big cities, where teenagers would learn work-skills from the age of 14.
Future mapped at 14
They say the scheme would raise the status of technical subjects and that students would also learn English, maths and science.
A university technical college is already in the pipeline, linked with Aston University in Birmingham.
The NUT's head of education John Bangs said: "There is a real fear about moves towards selection by provision, by direction and by the assumption that these routes are mapped out by these kids forever.
"How do you know at 14 whether you want to go down either the academic or vocational route?
"You are not ready to take that decision."
That was a view echoed on the conference floor by NUT executive member, Baljeet Ghale.
"Technical schools are not an innovative idea," she said.
"Those who failed the 11-plus scrambled for a place at technical schools to avoid being shipped off to secondary moderns.
"Comprehensive schools offer a chance for all students to achieve their potential."
Claire Mills, from Leicesershire, said: "A fractured two-tier system can only lead to a fractured two-tier society... keeping middle-class people for the more professional roles."
She said her parents had been told they would benefit from vocational education "because the chicken factory always recruits from the school".
Conservative schools spokesman Michael Gove said: "For generations Britain has failed to provide high-quality vocational education to match other nations like Germany or Singapore.
"A Conservative government would set up prestigious new schools offering a high quality technical education to any child who wanted it.
"They would not be selective but they would offer high-quality academic and vocational qualifications for students preparing for the world of work.
"A Britain whose future growth depends on manufacturing rather than booms in housing and banking needs to improve vocational education."
Mr Bangs was also very critical of the new Diploma qualification brought in by the government in 2008 which is meant to bridge the divide between academic and work-based subjects.
These are available for various levels up to the equivalent of A-levels and Schools Secretary Ed Balls has said he believes they could one day replace A-levels as the "qualification of choice".
Mr Bangs told journalists he "felt sorry" for the children currently taking Diplomas, because he said they had been "handled badly".
The union says it wants an integrated system, where vocational and academic qualifications have equal status and delegates voted to campaign for that.
The Liberal Democrats have said they would go further towards integrating the two areas and create a general diploma made up of GCSEs, A-levels and vocational qualifications.
The NUT's general secretary Christine Blower said: "We don't want 14-year-olds to be set on a particular path so that they have no opportunity to learn a foreign language or do creative subjects.
"We want parity between those who do vocational things and those who do academic things and don't want a sheep and goats situation."
The government denies bringing back a two-tier system.
A spokesman for the Department for Children, Schools and Families said: "This won't create a two-tier system - the Diploma is about bridging the age-old and damaging divide between vocational and academic education.
"All University Technical College students will study the broad mainstream curriculum up to 14 and then sit GCSEs, alongside more specialised courses, so they have a breadth of qualifications under their belts.
"The bottom line is this is about giving young people a far greater choice in what to study, according to their strengths. And by working alongside employers, it will give industry the well-rounded, skilled workers they are demanding for the future."