It doesn't make sense to measure achievement based on small age differences as a class of 25 to 30 students are never separated into small groups based on their birthdays. Should we even expect or want teachers to memorize each student's birthday? My kids are on the younger end for grade. I think achievement scores need to be grade-based rather than age-based because our students are sorted by grade into a specific classroom and sometimes sorted by their achievement levels into small groups within a single grade/classroom.
