I can advise as best possible. My wife and I are international children's rights specialists and have a broad knowledge of physical abuse, punishment and other 'criminal' physical acts. It is not corporal punishment but that is the closest I can get. In France it is lawful in the home and surprisingly it is not explicitly banned in schools, but is unlawful there nonetheless. Yes, I know that is a contradiction but unfortunately that is the present situation and needs to be resolved. However,this was not punishment but a harsh physical treatment which is unlawful. A sprained wrist is an injury, holding a head under water is not only abusive but potentially life threatening.The teacher telling your child he is not welcome and other British children having similar treatment is unlawful. Non-discrimination is strictly off the cards. On both counts the teacher should be in serious trouble.
France signed the UN Convention on the Rights of the Child on 26 January 1990 and ratified on 7 August of that year. It therefore passed into French law and here it looks like the law is consequently being broken.
Non-discrimination is :
Article 2
1. States Parties shall respect and ensure the rights set forth in the present Convention to each child within their jurisdiction without discrimination of any kind, irrespective of the child's or his or her parent's or legal guardian's race, colour, sex, language, religion, political or other opinion, national, ethnic or social origin, property, disability, birth or other status.
2. States Parties shall take all appropriate measures to ensure that the child is protected against all forms of discrimination or punishment on the basis of the status, activities, expressed opinions, or beliefs of the child's parents, legal guardians, or family members.
Therefore the Convention applies to all children, whatever their race, religion or abilities; whatever they think or say, whatever type of family they come from. It doesn’t matter where children live, what language they speak, what their parents do, whether they are boys or girls, what their culture is, whether they have a disability or whether they are rich or poor, foreign or French. No child should be treated unfairly on any basis.
The Committee on the Rights of the Child looked at this broader question of physical abuse in 2006 and concluded that it is the right of every child to protection from corporal punishment and other cruel or degrading forms of punishment in the sense of Articles 19 and 28, paragraph 2 and 37, inter alia.
Article 19
1. States Parties shall take all appropriate legislative, administrative, social and educational measures to protect the child from all forms of physical or mental violence, injury or abuse, neglect or negligent treatment, maltreatment or exploitation, including sexual abuse, while in the care of parent(s), legal guardian(s) or any other person who has the care of the child.
2. Such protective measures should, as appropriate, include effective procedures for the establishment of social programmes to provide necessary support for the child and for those who have the care of the child, as well as for other forms of prevention and for identification, reporting, referral, investigation, treatment and follow-up of instances of child maltreatment described heretofore, and, as appropriate, for judicial involvement.
Article 28
1. States Parties recognize the right of the child to education, and with a view to achieving this right progressively and on the basis of equal opportunity, they shall, in particular:
2. States Parties shall take all appropriate measures to ensure that school discipline is administered in a manner consistent with the child's human dignity and in conformity with the present Convention.
Article 37
States Parties shall ensure that:
(a) No child shall be subjected to torture or other cruel, inhuman or degrading treatment or punishment.
France is bound by these treaty obligations, let alone national laws, local regulations and teaching standards. I believe a maire is not up to the level you require to deal with this, nor a group of lay people. The starting point is the responsible inspector of schools, a dépot de plainte with the police. If the inspector will not act and the gendarmes appear unwilling then a formal complaint to the préfet by LRAR detailing everything, including particularly any lack of action. I would suggest cut and pasting the Convention articles above, looking for them online and then copying in the French version to give the impression you know exactly what you are doing and saying plus that you are deadly serious.
Upping sticks and going elsewhere does not prevent this happening again in the future somewhere else, nor does it save other children there from the same humiliations. It may seem like a bit of an effort but you will be doing possibly many children a favour throughout Landes at least. Best of luck and any further advice I can give you are welcome.