I used to do supply in challenging areas too. I always found the "chance card" raffle worked well with upper KS2. Any children displaying good behaviour got a "chance card" (done using raffle tickets by some people). At the end of the day they were all placed in a pot and one pulled out to win a prize. I usually had a selection of pencils, rubbers etc. and also found out the magic of Match Attax cards - not sure if they are still the "in thing" but something similar is worth investing in.