WhiskeyTangoFoxtrot
Super-curricular as opposed to extracurricular is activities/learning over and above what is expected on the GCSE/A levels course that directly relates to the degree subject that the student wants to read at uni.
For example, for medicine they will be more interested to see that you have worked in a care home or similar, for law they may want to see that you have volunteered in a law centre or CAB, for education they may want to see that you have worked with children eg. a a helper at Brownies etc.
Super-curricular would also include having won prizes or scholarship in the field such as engineering prizes that are available.
This is opposed to extra-curricular which in some way could be seen as past-times or hobbies such as music at a high grade, high level sports etc which although are indicators that the student is a well rounded individual as would volunteering in Africa etc/DofE. I am not saying that these are of no value to the student but not necessarily of value to their application.
In fact nowadays so many actually do all this it almost is the norm rather than the exception and becomes almost irrelevant.
The Personal Statement should show a real desire to study the subject for which they are applying.
As a Partner in a law firm my DH now looks for those who have had any form of paid employment whilst at school or during uni summer holidays as this would indicate a level of commitment to getting up, getting somewhere on time and earning their own money even if it is a job at McDonalds or cleaning at a school.
Going back to the very original question of the OP students who obtain good grades at a State school will be looked upon favourably to fill both quotas and the fact that they will not necessarily have been spoon fed like some students from independents might have. It will depend on which 6th Form it is though because some super selective 6th Forms would be treated the same way as independents would have. Also for those going private to GCSE and then switching to State 6th Form the unis do realise you have done this!