If it is an educational matter, then private schools can basically teach what they want. There is no cirriculaum in non-state schools.
It really depends on what the matter is about. Fair enough if you do not wish to discuss here.
If criminal, obviously the Police. The school, even though private, schould have a schools liasion officer (SLO) attached to it. They are busy plod mind, often having about 40 schools per officer.
MP's are best used after a complaint has been filed. They will not, moreover are not allowed to, interefere until the all proper avenues have been explored.
Has the school got a board of directors, if a private enterprise? If a PLC or ltd comp, they will have to have a secretary by law, full details will be freely available from Company House.
If affiliated to a church, then maybe the local bishop or parish.