How to get each girl to go to the festival with you:

Nei: answer shes my fren to sapphire in the hallway at lunch, then tell nei you love her.
Sapphire: answer tru in hallway at lunch or talk to her outside after school on the first day before dark. follow her instruction. then talk to her on the next morning outside.
Princess: get the black necklace (go in the school on the first day, after school, before dark, talk to the ghost on the roof, then the ghost in the hallway.) then talk to Princess in the classroom before school on the next morning and say yes.