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 inspectorate at the school board. Advice and support can also be found at the city, county or the state parent councils.
Landeselternrat Niedersachsen: http://www.ler-nds.de/