Charities Barnardo's Cymru and SNAP Cymru say too many pupils with behavioural problems are being excluded unofficially by heads who can't deal with them.
Parents are often asked to remove their child voluntarily from classes rather than the school imposing a statutory period of expulsion.
The illegal practice is blamed for a recent drop in official exclusion figures.
Both charities are meeting with Assembly government officials later this month to discuss the situation and call for more research into the practice.
John Sayce, assistant director of Barnardo's Cymru, said: "We are fully behind the drive to reduce pupil exclusions but schools should not be able to do it by the back door.
"`Unofficial' or `voluntary' exclusion is a term many headteachers use. However, it is clear that there is no basis in law for this course of action."
The report of the groundbreaking National Behaviour and Attendance Review (NBAR) in Wales called for an end to unofficial exclusions.
It said the practice can be seen by some pupils as a "reward" for poor behaviour that gives them unwarranted kudos among their peers.
The Assembly government's action plan in response to NBAR urged schools and local authorities to "eradicate" the practice, and said work would be carried out to assess the extent of the problem.
TES Cymru understands that a further announcement will be made in the near future.
Professor Ken Reid, chair of NBAR, said he welcomed the calls made by Barnardo's and SNAP and agreed with their sentiments in principle.
The Assembly government said it wants to see children and young people kept in mainstream education and as much as possible done to avoid them being excluded.
A spokesman said officials are talking to local authorities about information collected by the children's commissioner for Wales on unlawful exclusions.
There will also be a review of Education Otherwise than at School, including exclusion policy and guidance.