It is tricky because students often change their minds about what to continue in light of AS results.
It is also tricky because people taking the 1 year AS won't always be able to sit in with other L6th who are doing the full 2 years as what is taught in the lower sixth to the two types of student will not necessarily be the same.....so potentially a nightmare for timetabling. This makes it seem to me, that doing the AS for all 4 might be the only safe option, keeping all doors open.....then continuing to A2 with those who want to and arranging the teaching in the U6th to make sure everything g necessary has been covered....might just alter the order in which some of it would be taught, from the ideal.
I think many schools will go for this 'safe' option for the first year through and adjust as more information becomes available......ie will sit the AS exams at end of L6th in full knowledge that they dont count.
These wouldn't be seen as a total waste of time, because they will keep the students focused towards an external exam (in the old days the L6th was often wasted as pupils were not very motivated) and will also mean they are fully able to choose their A2s based on their AS results. Extra to this, there will be e benefit that everyone in the L6th follows the same specification....there won't be those following an AS course and those following different topics.
It is this last issue of the A2 course (what you would cover in L6th) not being compatible or the same as the AS course which seems to be the main concern.
Other people looking to NOT do the AS at end of L6th as a matter of course, how do you see your schools coping with this? Will the students have to say from the word go, which is their 4th option? Will they be timetabled just with other people only doing AS, rather than in with the L6th continuing to A2? I'd be very interested to know what people think about this.
In my subject (a humanity) this is a serious concern. I realise it might not be such an issue for something like Maths which builds progressively on skills, so the teaching in the U6th will always be dependent on what has been covered in the L6th.