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

Nei: answer shes my fren in hallway at lunch. basically just dont let her kill herself or get killed by the magic tiger.
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 2nd morning she says ur taking her to the festival. then talk to her after school.
Princess: get black necklace from the ghost in the school (2ways to do: go in school on first day after school before dark then wait til midnight and talk to ghost in hallway. or have magic tiger take care of nei after midnight during first night then enter school and talk to ghost in hallway before sunrise.) then talk to her in the classroom before school on the 2nd morning and say yes.