Number of 'A' levels typically taken. If you start with 4 can you drop to 3? And when?
Any option to change subjects if it appears (early enough) that a mistake has been made.
Additional opportunities to support subject - field trips, visits to universities, visiting speakers etc.
Support with applications for veterinary, medicine, Oxford, Cambridge etc. if likely to be a factor.
Sports available, even if just at a recreational level.
Opportunities to take on additional leadership responsibility - can be particularly relevant if joining a Sixth Form with a high proportion of internal students.
Uniform or dress code. General facilities.
Communication between teachers, parents, students. What, when, how...
How separate is the Sixth Form from the main school? Are the students treated as young adults with opportunities to be independent and 'grow up'.
What happens on 'Results Day' with regard to applications? What are the deadlines?