In the bug report I made a while ago that was merged into this one, and is still ongoing, is that even with the timed event (high school activities) is finished, the chessboard remains grayed out for most actions other than polish/reset the board and 'Join Game' which calls for the Sim you want to join in on a game, but the player Sim is unable to be a part of the game, even after 3 PM and into the next school day.
Even a Sim who's invited to the lot (who is not high school age (toddler/kid/young adult to Elderly) is unable to use the chessboard.
High School is the only active activity lot (outside of the Get Famous activity lot, where you could stay after the performance as an option at the end of the activity, though I never had done so, so I didn't test if chessboards have the same problem there too, I was usually running around trying to complete the activities there) that allows you to stick around, that's more than asking to do some overtime at the activity site, so I haven't been able to test this outside of the High School lot.