That sucks, hopefully you have photos of stones etc and neighbors would give statements and testify to this and previous incident, you probably best having word with CAB and police and if can go court do it yourself if can't get legal help, lot of courts quite obliging to individuals pursuing sensible grievances, if school that supervised how come they don't know children involved and had 3 incidents, seems it happened in school hours and on school property and not the first time the school got a role in it and have proven negligence in dealing with the matter by incidents happening over 3 occasions.
Hopefully a solicitor can give you some time or CAB can give you good assistance .
Shocking think this can happen and victim left out of pocket and with a huge task to get compensation !
Would of thought police would of been to school in attempt get names of who involved or have they ! .