The language offers are based on the level of knowledge of the pupils. There are special preparatory classes at regular schools. The children change to the regular classes after successful language acquisition. There are schools where children take part in a regular class and attend additional language courses. Please ask about the offers in your specific case.
For adolescents who are already 16 years old and whose education in the country of origin would expect them to be able to obtain the middle school graduation or the General University Entrance Qualification, there are special preparatory classes. This classes prepare pupils for a middle school qualification or the transition to upper secondary education. At the career entry school, there are vocational preparation classes in which language training takes place.
For additional information see https://www.landesschulbehoerde-niedersachsen.de/bu/schulen/schulentwicklung/sprachbildungszentren/deutsch-als-zweitsprache