You know, if the wizarding world experienced a baby-boom after the first war, the years that were 1-3 in HBP must have been huge (maybe ~150 students per year, if ~100 was typical before the war, based on the students of Severus' year sitting for OWLs). With all his unique duties (spying, watching Draco, watching Harry, brewing) he may have needed help reading those essays.