Definitely assign seats. Explain to them that it is so that they are
not blamed for the 'other' kid who didn't clean up. Then follow up in
the first weeks. Call the first kids back who don't clean up.
I don't think that the question is of multiple options, rather of
simple lessons. These kids have got to succeed with you in the first
weeks, doing 'your' lesson. We do a woolly woo thing in the first days
(idiot's guide to drawing?) and though it is a little humbling to admit
ALL are on board by then. Feels like two steps back but one is forward.