6th form is years 12 and 13. In year 12, you do AS (A-levels), then in year 13, you do A2 (A-levels but the second year)
It's not mandatory, you can choose to go to college, or go into employment. If you do want to get into sixth form, you have to get a required average grade at GCSE. In my school, it was an average of a B. I did an Engineering NVQ out of school as well, and that helped, so I recommend looking for extra things to do to improve your chances. Plus, you get paid for some NVQs
In sixth form, you get a lot of priviledges (in our school you get to jump the queues at the canteen, ignore the one way system in all the buildings and leave the school site), and you only do four subjects (in my case three, because I do an Engineering Diploma and that counts as two). Some lessons may be taught in other schools, in which case you get free taxis to and from the schools.
Finally, it's the best way of getting to university.
