Sixth Form or in fact nth form can be used to denote the year of secondary education. The latin master (Mr. William Charles Lucan-Roberts) who taught me while I was in Australia (prior to becoming a pommie) said that in the past it was typical to say 'nth form' as opposed to (n+6)th grade which is the current standard (in Australia).
For example, upon entering secondary school/high school you will be in the 7th grade, and in the 1st form.
In places like England the nth form notation is accepted standard, and the final (sixth) form, is split into an upper-sixth and lower-sixth. (Thus you end up studying for a total of 13 years, assuming you chose to stay on, and not leave school given the opportunity at the end of the fifth form. )