Upon the note:
"This is genuine pixie dust. Absorb it through the skin, and you'll get a dose of good luck. If you want more, find us at the shrine at the back of the school."
(The bags contain about 20 ml of dust, enough to mechanically give +1 to most rolls for 3 rounds, +2 for 2 rounds (if tier 1 or higher), or +3 for 1 round (if tier 3 or higher).)