The cashier from Tag, You're It is Angelita (Crybaby's friend in the Interquel, K-12).
It's said they are both played by Elita Harkov. So maybe, she knew Crybaby was like her and so helped her out. Hence their friendship in the second album/movie.
Or related to her, Vague Age is at it's highest when it comes to Melanie
Maybe the Cashier is Angelita's mom?
Kelly is a Spirit/Empath
Kelly seemed disapointed with Crybaby after Class Fight for not leaving a mark on her. Maybe she was mad because she was guiding Cry Baby
This may explain why Kelly stayed in K-12 at the very end
Also, how did Kelly get a snake in Class Fight? She has powers, of course!
In the same scene, Kelly woke up immediately after Angelita activated her Pyrokinesis. Maybe she could sense fellow Empaths?
Crybaby is slowly turning into a Spirit
Lilith's voice is very different from a human's and in many songs Crybaby's voice is altered too!