Retrieving Call Log History from iMessage Group Calls

My young daughter received numerous unwanted phone calls last night from classmates. Some of the callers had been blocked but they found they could still reach her if they called her via an iMessage group chat. As she blocked numbers, they tried calling using other unknown numbers. In an attempt to make the calls stop, she blocked as many numbers as she could and left the group chats. We also blocked any calls from unknown numbers in her settings. I went to gather evidence of the incident to show the school proof of harassment and bullying, but nothing is showing up in the call logs. There have been several incidents and it's time to take action... When she blocked numbers, all call history/evidence was deleted. Is there anything on apple's end that can pull up a history? It was about 25 calls that she repeatedly declined. I really need this evidence. These kids have found the loophole that they can reach her through group calls even if they're blocked, and there is no log. Kids like mine are left vulnerable and with no proof to show school/authorities.