Do you need to win against all 4 preds to get the ability to eat students? Or do you unlock the ability to play as the pred?
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You can play as Bella. Though when the students aren't the main pred, most of them are a fair bit shorter than usual.
Currently you can only eat somebody when your belly is almost empty - with the hunger meter at about 16% left.
Strangely, Tulpy seems to require a slightly lower % (e.g. waiting longer without food) than the other students, which is amusing to me. I suppose she is just uh... calorically-dense.