Michael Howard, Tory leader, announced this week that the party would set up a network of new schools to deal with England's most disruptive pupils, costing around pound;240 million.
The "turnaround schools" would cater for 24,000 pupils and replace existing pupil referral units (PRUs), which have places for 13,000.
Mr Howard said the new schools would differ from PRUs because children would get a full-time education and would be returned to mainstream schools only if they improved their behaviour sufficiently to gain a certificate.
The schools, which would mostly be run by local authorities, would enforce strict discipline and pupils would follow a curriculum based on literacy, numeracy and civic values.
Tim Collins, shadow education spokesman, said "turnaround schools" would be the Tories' top education spending priority. "It's important that we are calling them schools, rather than units, because they are not going to be dumping grounds or transit camps where young people only spend a few weeks," he said.
Teacher unions said they welcomed the idea of expanding places for disaffected students outside mainstream schools.
However, the National Union of Teachers warned that pupils in "turnaround schools" could be "ghettoised". Steve Sinnott, NUT general secretary, said:
"It could mean children were labelled for life."
Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, said that if private companies took control they might make the schools a "national network of boot camps for the disaffected".
PRU heads and academics said that the only significant difference between the planned "turnaround schools" and existing units seemed to be that they would take pupils further away from mainstream education.
Professor Sue Hallam, an expert on school behaviour at London university's institute of education, said that the most successful units for disruptive pupils tended to be run within schools rather than as separate institutions.
"The problem when you make PRUs separate from schools is that pupils have to travel further, which has a negative effect on their attendance, and it can become more difficult to reintegrate them," she said.