All councils will be surveyed in an attempt to discover the patterns of exclusion and return to school across the country. CEOs believe that local authority schools are being forced to take in more than their fair share of excluded pupils.
CEOs claimed that GM schools had already weeded out children through their selection policies, and suggested that they should not be excluding as many pupils as they were.
Government figures on permanent exclusion show that the opt-out sector is excluding proportionally fewer pupils than LEA schools. But CEOs believe that parents of pupils in GM schools have been urged to withdraw their children from school "in order to avoid exclusion".
Robin Squire, junior education minister, denied such a practice existed.
But council officials said they had received reports of it happening in Kent, particularly in the south of the county where pupils whose parents had agreed to withdraw them found it almost impossible to find another school place.
Statistics on exclusions in Kent dating back to September 1994 show that for every 10 children excluded from either local authority or GM school, one was returned.
But they present a different picture when the county is broken up: in west Kent, GM schools excluded 55 pupils and took back 10 (a return rate of 18 per cent) while LEA schools excluded six and reinstated two (a rate of 33 per cent). In all parts of the county, except north and mid-Kent, the return rate to LEA schools was higher than that to GM schools.