HINT/SPOILER!
The locker key uses a mathematical premise many might not know, but otherwise really nice game.
Anyway, I had to Google it, but basically one has to find the value of the A, B and C of the poster-puzzle, and for some reason it is her grandmother's locker combination? Idk man.
<a href="https://layton.fandom.com/wiki/Puzzle:Letter_Calculations#Hint_3" ]<db143]="" letter="" calculations="" |="" professor="" layton="" wiki="" fandom<="" a="">[DB143] Letter Calculations | Professor Layton Wiki | Fandom</a>
^Use this link to understand and calculate the value of the letters.
But still, cute game, cute story and I absolutely love the artstyle.