Schools where more than half the pupils are new every year need more money to help them cope, according to a report published this week.
A typical secondary pupil arriving mid-year takes 29 hours to settle in at an estimated cost of pound;800, while a primary pupil takes 15 hours, costing nearly pound;400.
The research was conducted by the Association of London Government from data provided by 67 primary and secondary schools in 23 boroughs.
Schools said achievement was affected in classes disrupted by new pupils needing extra help.
The most common "mobile" pupils are refugees or asylum seekers from traumatic backgrounds, or those housed in temporary accommodation because their families are homeless or fleeing domestic violence.
Other "mobile" pupils are those who have been excluded, or who move schools before they have the stigma of being officially excluded. Many of these pupils have severe behavioural problems.
Sir Robin Wales, ALG chair, said: "Schools and teachers are already stretched to breaking point. More funding is needed to help schools cope and ensure that London pupils get the same chances as other pupils."
He also called for more permanent social housing to be built by the Government for the 73,000 London children living in temporary accommodation.
The research showed that schools already facing great challenges were often those who admitted high numbers of mobile pupils - because they had spaces available.
Inner London schools receive on average 14.2 per cent of their pupils mid-year and outer London schools 6.8 per cent, compared to 5.7 per cent in the counties, 5.9 per cent in other metropolitan cities, and 6.3 per cent in other areas.
David Hart, general secretary of the National Association of Head Teachers, said mobility was a serious concern for many of his members. "I would certainly support an urgent investigation into how mobility affects schools in all parts of the country, not just London," he said.
At present there is no specific government funding for students who change schools mid-year.
A spokeswoman for the Department for Education and Skills said: "We have been considering whether there should be a mobility factor in the school funding distribution formula, to reflect the fact that schools in some areas face higher levels of pupil turnover than in others.
"Ministers expect to announce their decisions on the new school funding arrangements, including on the mobility issue, in the summer."
The report, Breaking Point, is available at www.alg.gov.uk