The group, known as the Kingswood Partnership, is piloting a radical approach to the curriculum which aims to give students the time to develop research, citizenship and thinking skills.
Between the ages of 14 and 16, pupils will still do traditional GCSEs. But alongside them, around a sixth of curriculum time will be devoted to a cross-curricular "personal challenge".
Pupils will be able to choose, for example, to do a research project allowing them to broaden their subject knowledge in a particular area. Or they could do a specialist, or work-related course.
Students would gain a special Asdan (award scheme development and accreditation network) qualification for this work, which is recognised by universities.
Another element of the partnership's work will be the development of "individual learning plans" for each pupil, which will give them targets in maths, English and specialist courses, and will be accessible to parents by computer.
Kingswood is one of 25 Government "pathfinder" partnerships between schools and colleges, which are trying out new ways of providing education for 14 to 19-year-olds.
Cross-curricular work, thinking and research skills are likely to be major parts of any diploma system. But Kingswood's scheme is not a fully-fledged diploma, as students still have to be offered existing GCSEs, A-levels and specialist courses.
David Turrell, manager of the project and head of Sir Bernard Lovell school, one of those taking part, said that it had been difficult to find the extra time in the school week for the "personal challenge". Taught time was being increased.