Raised Beds - On Concrete

Our local primary school really got into growing vegetables this year and have asked me if I could make them some extra raised beds. The problem is that they've asked for these to go on a concreted area of their playground. A couple of questions then :? ;

I was thinking of going to the local timber recycling company and getting some old scaffold boards - 1/. would these be safe? (any nasty treatments?) and 2/. would two high be enough?

I'd have to line the beds to stop the soil washing out but also need to make sure it drains well as primary school children like watering! Any (cheap) suggestions?

Finally, does anyone know if ISPM 15 treated timber is safe for use as a compost bin?

Thanks