Usually it would be the year you finished 12th grade, no matter at which school.
So for me for example, born 1968, I am in the class of 1986. (When I look at my report cards however, I realize they should have definitely made me repeat a year, putting me in class of 87. So add a year if you repeated a year.)
Born 62, class of 80
Born 63, class of 81
Born 64, class of 82
Born 65, class of 83
Born 66, class of 84
Born 67, class of 85
Born 68, class of 86
Born 69, class of 87
Born 70, class of 88
Born 71, class of 89