South Manchester high school, in Wythenshawe, serves a large council estate and the vast majority of its pupils come from white working-class families.
The area suffers from high levels of unemployment, poverty and teenage pregnancies.
Nearly half the pupils have free school meals and 40 per cent are registered as having special educational needs. The local population is very transient with around 20 per cent of pupils leaving the school every year.
Phil Taylor, South Manchester's headteacher, said: "Maintaining morale in inner-city schools is hard enough without initiatives like this which send the message that we are failing our students.
"Our brightest pupils regularly get 10 A-Cs at GCSE - how many members of the Cabinet can say the same?
"If we can achieve these standards then we know there is nothing wrong with our teaching. What kind of message does this send about social exclusion when the brightest children are to be creamed off and sent elsewhere? Comprehensive education is supposed to be about trying to do the best for everyone."
"This Government obviously knows nothing about school timetabling. It is simply not viable to have children coming and going out of school.
"Tony Blair has been going to too many Islington dinner parties.
"The kind of parent he is talking about is looking for any excuse to send their kids to a more privileged and elitist school. This will do nothing to change their minds."
Pierre Grace, head of science, said: "The extra money needed to be targeted at all pupils in inner-city schools. We already have a problem attracting resources to the school and buying textbooks and information technology equipment. This will do little to improve the situation.
"If anything it will move the resources to another site and only the top 5-10 per cent will benefit."
Dawn Phillips, special needs co-ordinator and English teacher, said: "This seems to contradict Manchester's policy of inclusive education if the brightest children are to be sent to another school for extra help. We are not going to see an influx of middle-class children because of these changes.
"Local parents already have faith in us to do the best for the children. We have some very bright kids and they do well. But we also have lower-ability children with poor literacy because they do not learn pre-reading skills or get support at home.
"I will have to wait and see how this plan will help them."
Gill Bridge, deputy head, said: "In our experience it is very difficult to encourage our students to take any academic activity out of school hours. They may stay for music, sport or dance but they will not stay and study.
"The problem of poverty cannot be underestimated. From 14 onwards most of our children have part-time jobs and if we set up a programme that forces them to choose between their job and their studies they will choose the job. These families have no spare money and pupils' jobs often represent all their personal income.
"Also, our pupils are extremely insecure young people. Even the bolshy ones quickly lose confidence in an unfamiliar environment. It is unlikely that they would be willing to go to a 'posh' school to get extra lessons.
"Our bright kids do very well here. I do not see that they will gain from getting more of the same elsewhere."