School invoicing for broken ipad ? fair or not

Dear all

I just want to test the waters with this one. My 6 year old son is in mainstream school with an EHCP and 1:1. He has profound sensory issues and when dysregulated can bite done on hard objects. This rarely happens and has not happened in school until last week, when he bit down on an ipad. The response of school is to invoice us for the cost of replacing. Is this reasonable. To me it feels inherently unfair as the damage was not intentional and a direct result of his dysregulation. Furthermore I do not have an understanding of what happened before to lead to this outcome?? 

I have told the school that I think an investigation of what happened should be there immediate course of action rather than asking for money. They disagree.

Any thoughts welcome..

Thank you