Most of the secondaries around here do not have a sixth form, although DS's does. There is a medium sized sixth form in town and one of those warehouse-type sixth forms approx 15 mis away by train.
The (surprisingly, large % of) DC who are leaving DS's school, are mostly looking to go to the warehouse place. Their mums say to me that they are just "ready" for a move, similar to when they had outgrown primary school. TBH, I find it an odd decision. Why would you leave a sixth form that gets far better results, and has really good pastoral care to go a warehouse with weaker results and is a train ride away? (I just nod and smile though!)
Others go private for the excellent career advice and help with university applications.
DS is leaving (if he is leaving) because he has a place at a sixth form that will give him a highly specialised education in the area in which he excels and no regular sixth form could do that.