You have the homework bug.
"11. How to fix homework bug. "
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After fixing it you must progress in the mentor quest for a very long time plus you must work on all open quest and some closed quest at the same time and get both Lisa and Alice's mood as high as it will go and keep it there from now on in this game.
Next time you play and as soon as you offer Lisa to help with homework for free you must get her spanked within a few days or you will get the homework bug again.