Can't have it during the Easter Holidays ~ schools wouldn't get the rake off commission on the sales would they?
As for the blurb that Nick has found ~ sums it up nicely ~ the whole thing is a con to get the kids to pester their parents into buying books at twice or three time the price you would pay on Amazon for the same book and the schools get a kick back. Every school taking part in this will have someone there selling books and the £1 voucher will not buy anything. I have never seen a book for less than £5.99 at one of these events.