My child accidentally broke a sprinkler head in a locker, while he was left unattended at school

You don't specify how old your child is or whether this was a public or private school. As a general matter, parents have no civil monetary responsibility for the negligence of children who are not under their supervision. It sounds as if the time of the accident in question, you child was under the supervision of the "plaintiff." That's not a good fact pattern for them. Just say "no." If a lawsuit ensues, contact your homeowner's insurance carrier.