The sixth form in a British school consists of the classes that pupils go to from 16 to 18 years of age, usually in order to study for A levels. She was offered her first modelling job while she was still in the sixth-form. the highest level in the British school system. Children aged between 16 and 18 stay in the sixth form for two years while they prepare to take A levels (=the highest level of school exams) . former