UK school years
We used to have years 1,2, 1,2,3,4, 1,2,3,4,5,6th form.
We then changed to 1,2,3,4,5,6,7,8,9,10,11,12,13 (years 12 and 13 are 6th form, no one know where this name comes from).
Then we added reception year, to get R,1,2,3,4,5,6,7,8,9,10,11,12,13.
R is just a different symbol for 0. (In the US K is used for 0.)