Yeah, you have to check on Violet (at least if the player doesn't skip Violet's optional scene in the Club) in order for the "sell the jar" task can become active. After it becomes active, Sara should be in the classroom in the evening. In my video walkthrough (
#25,509) after checking on Violet it pops active but before seeing Sara, I have the MC go to the comiccon to get rid of all those tasks. After the MC sleeps the night, the following evening is when it worked in my playthrough.
I just checked about the MC seeing Sara the same day it goes active and again had no problem. During the morning and the day, Sara tells the same "lunch time" message to the MC but in the evening she takes the jar.
So I know the same day or the day after works... People shouldn't be able to get into the school on Sunday because there is no school on Sunday, but in case they can get in she won't be in the classroom - only Mondays-Saturdays are possible.