Sixth form is a term used in the British education system; it’s the last two years you spend in high school. For US students, that’s Junior and Senior year of high school, and for Indian Students, thats Class XI and XII. The reason its important is because British students choose their A level subjects.
Some quick context: A levels are the major external exams done at the end of your last year at high school, British students study for them for two years. Students in the British system choose 3-4 subjects, and study those exclusively for these last two years. These grades are really important, because they play a big part in your college admissions, and sometimes are even looked at later in life.
Sixth form is also an important time for British school students in the UAE because it represents a change in school. A notable number of teens move schools at this time, because it’s important you have the optimal fit for you in these last two years. That means we say bye to many good friends, and meet new ones as people move from place to place. In my year, about half the year is moving schools (including me).