Legislation by the Government last year appears to have succeeded in cutting the proportion of excluded pupils who are reinstated by appeal panels.
But the National Association of Schoolmasters Union of Women Teachers reports that headteachers are now seeing a growing number of expulsions overturned by governing bodies instead.
Members of the union in eight schools are expected to begin voting today on whether they should refuse to teach reinstated pupils. NASUWT staff in four other schools agreed to take such industrial action last month.
Of the 12 pupils at the centre of these cases, only two had their exclusions overturned by appeals panels, with the remaining 10 let back into the schools by governing bodies.
Chris Keates, deputy general secretary of the NASUWT, said she found the trend "disturbing".
In two of the cases, the union claims the governing bodies decided it was unreasonable to expel the students because they did not have a history of troublesome behaviour.
One of the pupils had attacked two teachers in a space of 24 hours, while the other had been caught drug-dealing.
Ms Keates said: "In some cases the local education authority was telling governors not to exclude because they did not know where to put the student.
"In others the governing bodies have made the judgement because the student did not have a history of problem behaviour. But headteachers should be able to exclude a pupil for their first offence if it is serious."
Neil Davies, chairman of the National Governors Council, said governors were only legally able to reinstate pupils if exclusion procedures had not been properly carried out by heads.