Fair point. I would use the age.
However, I discovered about a year ago that the terminology for school "years" in the UK seems to have changed and follows a similar system to AmE.
When I went to school, I was in "5s, 6s, 7s, 8s" at "first school" (now called primary school), then I was in "1st year/2nd year/3rd year/4th year" at "middle school" (now called junior school and which now finishes at the age of 11, not 12), then in "2nd year/3rd year/4th year/5th year" at "secondary school/high school", then in "Lower 6th/Upper 6th" at Sixth Form College.
None of those terms seem to have survived.
My work colleague, who has an 11-year-old son, tells me that all schools now refer to their students as being in "Year 1" (age 5) all the way to "Year 11" (age 16). From 2017 (I think), all students will be required to stay at school until the age of 18 so presumably, they will enter "Year 12" and "Year 13" before they can leave school.