Basically, you have a free choice at which school type you want to register your child. Pupils aged 16-18 should seek advice on which type of school is suitable for them. The state school boards (http://www.mk.niedersachsen.de/startseite/wir_ueber_uns/einrichtungen_und_behoerden/niedersaechsische_landesschulbehoerde/niedersaechsische-landesschulbehoerde-6464.html) help with consultation.