During the Schools Out Event.
That is if you had him in there and not turned in to Mrs Robinson before the Event started.
But when I returned one to the square above the Fairgrounds I seem to have lost the other one too even though the quest says still active, it still appears in the maps 'show active quests' feature. Will have to see what happens when the school is open again, LOL.

p.s. Yes, I have many quests ready to turn in but not turned in. As a matter of fact I have them all, all except the 2 statue ones which I do every day while I'm playing (well that's only during this event). That's a total of 50 repeatable daily (well some not so daily) quests.