In the demo, student predators won’t do anything unfortunately. Even if you already beaten Bella, there won’t be another Bella as a student predator, since Bella is the only predator you’ll be facing against in the demo.
This is the same case in the full game as well: In a fresh save, if you already beat Bella, select Student Predator as an option, and then go against Bella again, there won’t be another duplicate of her as a student predator if the Apex Predator of her already existed.