Complex sentence: She then gives them the ultimatum that if they admit that they have been cyberbullying Vanessa, they would only be suspended for a week, but if they don't confess they would be expelled and not graduate with their class.
Split into simpler sentences: She then gives them the ultimatum that if they admit that they have been cyberbullying Vanessa, they would only be suspended for a week. But if they don't confess, they would be expelled from school and will not graduate with their class.