Switching schools is disruptive to pupils' grades.
Results in English and maths for children at Key Stage 2 dropped 12% following one in-year move, 17% for two moves and 25% for three moves.
The report found that during 2011-12, there were a total of 300,000 in-year admissions.
This means that for every 10 pupils who moved from primary to secondary school in September 2011, another six moved schools during the school year. a But the RSA warned that the volume of in-year admissions may be about to increase as a result of changes to housing benefit, leading people to move from high cost to lower cost areas, and an increase in the number of young people in care.
In-year moves are more likely to be clustered in lower attaining schools and in more disadvantaged localities, the report found. Examining data from the National Pupil Database, the RSA discovered that a child who changes school in-year due to moving house is three times as likely to move to a low performing secondary school as to a high performing one.
RSA director of education Joe Hallgarten said: "Our research found that in-year moves are highly disruptive to children, badly impacting on the social and neighbourhood relationships that can support their schooling. Pupils who move in-year of-f ten can't access high performing schools that have no spare places. And unfortunately it's ' the poor who suffer most - with pupils who move in-year coming from families with more challenging home circumstances."