Certainly a good idea in principle, if exam time has passed, but has to be correctly managed. Wearing old clothes is one thing: will the children be briefed on what to be wary of?; will each manageable group have a "supervisor" of whom they can enquire, and who can deal with "problematic" items?; are suitable "grabbers" and disposable gloves being provided? If yes to these, then it is an idea which could do a lot of good and find widespread application. Any acceptable means of reducing litter is beneficial healthwise and for local finances. Not sure about the detention though, and pupils who may have physical problems need to be involved in some other community activity -maybe, after the work has been done, collating a summary of what was collected or risk being "labelled" - you know what I mean. And no - I'm not in Teaching or Health and Safety!