In Britain the first high school graduation happens after 11th grade; attending sixth form is optional and is primarily done by students intending to study at University.
In these years you specialise in a couple of subjects relevant to your intended course of study, and for university you apply and are accepted for and study exclusively one subject from day 1.
So arguably the US equalivalent is the freshman year of college.
Just to clarify this as well, while sixth form (17~18 years old) is optional in the UK, education is still compulsory until you're 18. you have the option to do this at an apprenticeship or skills based school but lots of people do just default to a levels.
> in the UK, education is still compulsory until you're 18
I've just looked it up because I hadn't heard this - it was only compulsory until 16 "in my day"! Turns out it still is, here in Wales, and also Scotland and NI. Only England changed it to 18. Our devolved governments love to make things confusing.
It's certainly a real term. The context is often student athletes that intentionally didn't play their sport for a season (called redshirting) to maintain their 4-year eligibility so that they can stay for a fifth year and compete in their sport.
We tend not to have a 13th grade, and when that does exist, a PG (post grad) is generally there because they excel at a particular sport.