That sounds an excellent idea!
I can't really offer much advice - dd's school sells off second hand uniform now and again - nothing regular. I don't know who is donating it though; I don't recall being asked for any contributions.
With three children I am certainly not adverse to buying second hand clothes/uniform if they are in good condition and often scour second hand shops and have found some fantastic buys.
However, one thing I would say (and this is really just me, my problem totally I'm sure) but on the occasions I have seen it on sale at school I have felt a bit um, uncomfortable shall we say rooting through the clothes.
The last time, I seemed to be the only mum having a look and wondered if I came across as a bit, well desperate (I'm not, but I'll happily have a bargain!), or maybe that's why the others were steering clear? I really don't know the reason, but I did suddenly feel uncomfortable with it and stopped. It was in the main hall and other things (cake sale etc) were in full swing with a huge gap where the clothes were. It's silly too because proceeds were going to the school.
As I say, probably just me being oversensitive!
It was strange though, and I made a mental note to give it a miss next time. If there was a way to buy it/ know what was on offer without going through that again I would be interested.
Don't want to put you off because it's a great idea. I don't know how you will be running your scheme, but something to perhaps consider.