Those statements arguing that institutions run by religious orders have complete freedom to do as they please are exaggerating their independence. The bishop could revoke the order's right to operate any "Catholic" educational activities within the diocese on the grounds of posing a danger to the faith and well-being of Catholic children. Since the incident involving the pro-abortion teacher was a matter of public scandal, this expulsion could hardly be designated a purely "internal matter" of the order of nuns.
He could, but he won't. The order likely owns the plant and land, so nobody else could walk in and take over the school. Diocesan high schools couldn't absorb the students if the order was expelled.
This may not be the first run-in this school has had with Ms. Sills.