One of the things that our PTA does at my daughters Primary School is a Happy Bag collection. My daughter brings home a bag which I fill it with second hand clothes. We then take it back into school on the day of collection, these bags are weighed, and according to the weight our school gets paid. The PTA raises some money and we get to recycle our clothes - win, win.
If your school is not a member of this - then I would recommend getting them on board - more details can be found at http://www.happyschoolbag.co.uk/