That's definitely unfair without question. I'd be frustrated if all my hard work went to waste. It should be upon the university to investigate it thoroughly, and find the students no matter what rather than take the easy way out and penalise everyone. It would have been way better if, as someone said, they do a new assignment, and if they also find the perpetrators, hand them the well deserved punishment and set a deterrence the right way. It reminds me of primary where a few students would talk and disrupt class, and then we'd all have to stay in for recess. This is just way bigger, and more unfair.