CRGS holds a sixth form open evening in the autumn term , this years is 20th Oct. Application packs are handed out on the evening. Offers are based on predicted/already achieved GCSE A* grades and spaces available in subject choices. An interview is held to discuss options/ flexibility in subject choice. Make sure he is sure of his subject choices at this stage, we tried to change after GCSE results but were unable to, due to class sizes.
Be aware, progression to year 13 is not automatic at CRGS, several students failed to make it this year.
However, it is a very small minority who are not allowed to progress and its very unlikely to come 'out of the blue'. If your DC is admitted as an external entry and works hard and makes the most of the help available it is very unlikely they would not do well enough to progress to year 13.
Absolutely, its not automatic but neither would I say is it something to worry about if you are capable of the results that are necessary to gain entry.