In case of problems at school, you should always first contact the class teacher of your child. Together, you can advise on what to do. The class parent speakers are also contact persons, if you need support. If you cannot solve the problem with the class teacher, you can contact the headmaster. If you cannot find a solution here, you can contact the responsible school supervision in the state school office in your region.
State school offices in Niedersachsen: https://www.landesschulbehoerde-niedersachsen.de/organisation/standorte
Service points of the state school board Niedersachsen: https://www.landesschulbehoerde-niedersachsen.de/organisation/servicestellen
You will also receive advice and support from the parents’ association:
Landeselternrat Niedersachsen http://www.ler-nds.de/