The National Union of Teachers was balloting members at Norlington School for Boys in Leyton, north-east London, this week over possible strike action.
Education analysts believe more teachers will ask to teach part time in future, as a wave of younger teachers hit their mid-30s.
The Norlington dispute involves Lee Boyce, 34, its head of English, who is returning to the 580-pupil school in September after having a son, now 10 months old.
She wanted to return to her post for three days a week, sharing the job with another teacher. The school offered a compromise of part-time work: a four-day week.
Rinaldo Frezzato, Waltham Forest NUT branch secretary, said Ms Boyce could not accept the offer as it would mean doing five days' work in four.
"Very few heads understand the difference between part-time working and a job share," he said. "If you come back from having a baby and take part-time, you are stalling your career - possibly for good."
Employers are obliged by law to consider applications for flexible working, and must give a good reason if they reject such proposals.
Mr Frezzato hoped a strike would not be necessary, but said the action would "send a message" to schools.
Waltham Forest council said it was in talks with Ms Boyce and the union, but the matter had not been resolved as The TES went to press.
Chris Robbins, Waltham Forest's cabinet member for children and young people, said: "While it is regrettable that this ballot is taking place, I hope this matter can be resolved without disruption to vital education services."
John Dunford, general secretary of the Association of School and College Leaders, said as long as the school was within the law, it should not be a striking matter. "I hope it does not set a precedent," he said.
Professor John Howson, a recruitment analyst, said the proportion of jobs advertised for maternity cover rose from just under 5 per cent in 2006-07 to 6.3 per cent in 2007-08. He said it could be harder for secondary schools than primaries to accommodate job shares because of timetable restrictions.