We had this one too Yorkiegirl; PO are very peeved about it, and ours arrived with postage to pay. We went and picked it up at the PO and paid for it, and were very cross when we saw what it was. It was from a friend of ds1's (in yr2 at the time) but had clearly been written by a parent.
We actually took it in to the school, and passed it on to the Head teacher, who did an assembly based on it, and the fact that it was 'cheating' people, and the PO.