Dumb, the blackmail is weak at best
The whole point is that she gets desperate over the slightest misdemeanor 'cause her mom is implied to be an abusive control freak (flashback with MC saying she'll be better while crying onto 96% tests + getting calls over being late to
a cramschool)
That's also the reason why she got the urge in the first place.
So while it's stupid, this stupidity has tangible consequences.