My Dad's first primary school graduating class from Scoil Mhuire, where he was principal for 14 years, threw him a surprise party on the occasion of the school's 20-year-anniversary. They are now men and women in their mid-thirties. He didn't remember all of them, but as he said, if he didn't remember you, that meant you were a good student who didn't get sent to the principal's office a lot.
Here's the bit in the local paper. He was chuffed!