I've just put my order in for next years school clothes at Marks and Spencer and thought I would pass on this bit of advice.
At first I put my whole order through at £92, after the 3 for 2 discounts it brought the total down by £14 to £78. As it took the cheapest items off 3 x £3 and 1 x £5.
I then started putting the order through again in sections (total 4 orders), braking the orders down allowed me to put the £12 items through as 1 order, the £6/£5 items through as one order, the £4 items, and finally the £3 items. This gave a total of £29.75 saving, an extra £15.75 on the whole order.
Using the voucher code for free delivery SCHOOL09 meant the amount of orders placed didn't matter as it didn't incur further delivery charges.
So put your orders through seperately peeps and save!!!