Pregnant girls should not be seen as a health and safety risk by schools worried over pupils going into premature labour.
Collette Ryan, an inclusion specialist, said expectant mums should stay on longer than six months, the time she claims most schools pressurise them to leave.
She also says schools should stop treating teenage pregnancy like a bad behaviour issue. Her research found that girls were being sent to pupil referral units to continue their education, simply because they were pregnant.
Ms Ryan, the manager of the inclusion support centre at Ysgol Emrys ap Iwan in Conwy, said the treatment of teenage pregnancy by schools was creating a stigma, as well as denying girls their educational rights.
"My argument is that teaching staff can be pregnant at school and allowed to go almost full term, but teenagers are being sent home for health and safety reasons."
Ms Ryan also said some heads wanted girls out when their bumps became too visible because it gave the school a "bad image".
Inspectorate Estyn recommends pregnant teenagers should be encouraged to stay in mainstream school as long as possible to give them the best education. Schools are encouraged to make allowances, such as allowing the girls to leave a lesson five minutes early to avoid the rush hour.
However, Assembly government guidance permits heads to ask a pupil to leave if they feel school is inappropriate.
Gareth Jones, secretary of the Association of School and College Leaders Cymru, said he believed schools were generally supportive of pregnant pupils.
He said: "As long as they carry out a thorough risk assessment it should not be a problem."
Specialist promotes the joys of inclusion
Collette Ryan believes the special inclusion unit at her school could never be described as a "sin bin". She thinks a similar unit for pupils with behavioural or other social problems - excluding pregnancy - should be available at every secondary school in Wales.
Six years ago the school had a bad behaviour record and rising exclusion rate.
But earlier this year, inspectorate Estyn described the inclusion centre, which is based on the site of the school, as an example of outstanding practice in tackling bad behaviour.
"Pupils have a supervised lunch and break and that's the most punishment for them; they miss the social side of school and want to go back," she said.
Ms Ryan's call for more inclusion comes as schools with hard-line exclusion policies are under attack for increasingly sending pupils home for trivial reasons.
Neil Foden, head of Ysgol Friars in Bangor, is known for his school's strong line. Speaking on substance misuse, he said exclusions often deterred pupils from re-offending.