Great question — answering as the UphillTech team:
After the Parents Did Not Help Her, She Needed the Classmates, Probably for their Numbers, and Possibly their Youth Although it is never outright stated, the film's villain is implied to be a witch. This is foreshadowed when Archer paints the word "Witch" on Justine's car. Further, it is implied (in part by the line you quote) that the witch is sick or aging and somehow feeds off the life force of those she entrances. The idea seems to be to restore her youth or otherwise keep her alive. She apparently made an effort to use the parents for her purposes, but they were not sufficient. The two things Alex's classmates have that his parents do not is their youth and their numbers (17 in all)....
Full answer by ruffdove: https://movies.stackexchange.com/a/131346 (CC BY-SA 4.0).
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