You have to fail the test twice.
1. Fail the test twice on Friday (by not doing homework).
2. After the second time failing the test, talk with Teo (should have an exclamation mark on top of his head) and get his quest to steal the test answers.
3. Go to school on Thursday at 7 p.m, and use the ladder on the left. You’ll find Mabel stuck. You help her and fail for now, going back home.
4. Do the same next Thursday and climb the ladder. Enter the school and go to the second floor.
Problem is, while in class at present time, Theo is only bringing up the lesbian magazine no matter what day it is. I would think after the dialogue for it ends he should be bringing up what you are referencing here but he's not.
Developer needs to look into this as quests shouldn't be locked to a single one at a time.
In this case, I may possibly need to obtain 4,000 for Vol's 2 & 3 before I can proceed with Mabel's questline which is just plain dumb when they have nothing to do with one another other than Theo being involved with the both of them. 😡