The usage depends entirely on the country. 'Grade' is used in the USA. An American student in the 12th grade would be aged 17 or 18 and be in the final year of secondary education. 'Grade' is never used in the UK. In the UK, 'grade' is a level or a mark, not a year of education. Nowadays in the British state sector, the terms Year 1, Year 2 and so on are used. Reception: age 4-5 Primary: Years 1 - 6 (ages 5 to 11) Secondary: Years 7 - 11 (ages 11 to 16) Years 12 - 13 = Sixth form (ages 16 to 18) In some private schools, the older terms 'first/second year' and 'first/second form' are still used, with 'first form' being the equivalent of Year 7. The term 'Sixth Form' is a relic of this system which is still used in both the private and state sector.

In Northern Ireland (UK), primary school classes are as follows: P1 (for the first year of primary school), P2, and so on until P7. Mainland UK is slightly different, as they have a "reception year" before primary school starts. Then, you go to secondary school, and the years are called "First year", "second year", and so on until "seventh year". Sometimes, "sixth year" is also called "lower sixth" and "seventh year" is called "upper sixth". In secondary schools, the years can also go by the number of years you've been in school, i.e. "first year" can also be called "year eight", "second year" is also known as "year nine" and so on. This is generally used in the following context: "The students of Year 11 are going on a school trip next month", but you could also say "the fourth years are going on a school trip next month". Using the term "grade" is American and not used in the UK. I hope this helps!