Outrageous that the head should be encouraging children to lie to their parents. AND that the LEA should be so lax about it - how dare they refuse to tell you what the guidelines are?!
I suspect SS and Ofsted would take a dim view of a childminder, for instance, who didn't bother to tell the parents that one of their charges had hit their head when a bus collided with a car. Can't see that it's any different for a school.
Years ago my little sister's headteacher tried to hush up an actual proper coach crash during a school trip. Apparently all the (primary age) children were given very strict instructions that they were NOT to say anything. As if the parents wouldn't wonder why they were two hours late back...