I have been thinking about this too. I am thinking the more natural split of 7,8,9 and then 10,11, sixth form. But this is because one of my HLTAs oversees exam access arrangements so would be helpful for her to oversee 10 and 11. I think by need would be very difficult due to overlap of numerous need and depends on the % split in your school. interestingly i was going to go deputy SENDCO rather than SENDCO!