Please don't bother to send your child in ...
Professor Ken Reid also claimed that the technically illegal practice, which has been adopted by some headteachers, appeared to be worsening as school reorganisation brought with it the threat of closure for those with falling pupil rolls.
His comments follow the publication of a report by Maria Battle, the acting children's commissioner for Wales, about the potential dangers of "hidden" exclusion policy.
It cites the case of 13-year-old Luke Tanhai, a pupil at Cardiff High School four years ago, who was unofficially excluded several times. He died in an accident with a bus on the day he was permanently excluded. The school claimed it had met all procedures,
But this week his mother Hannah Tanhai condemned the school's actions on the BBC Wales programme Week In Week Out.
"I didn't question anything for so long," she said. "They made me feel what I was doing was right, but now feel I let him down."
Professor Reid, deputy principal of Swansea Institute of Higher Education, said the report would go before a group of experts late next month.
"We were already aware of unofficial exclusions," he said. "But I think everyone has been surprised at the extent of it."
Maria Battle, acting children's commissioner, is calling for clearer government guidelines for schools and local authorities, as well as informing children and parents over their rights.