Does the director allow the band to slack off? Also another thing to think about: suggest running intense rehersals, they cant slack off if they don't have time to slack off. Keep the rehersal moving and don't slow its pace. If you are running through a set, If its bad, have them run back and do it again. Don't give them time to slack off in between sets. Always tell them to run. On the field you should be doing one of 2 things- running, or marching. Never have the band walk anywhere on a field.
And as for the poor marching. Do you guys practice basics? by basics I mean have the whole band in a block at attention and march forwards, backwards, left and right. You should try to do this before every rehersal to drill these basic marching skills into their heads. Also, Marchng the whole show with no music, only focusing on drill, a few times every practice could help improve marching.
Also, When they act up, have them drop and give 20. Thats the most common way to treat missbehavior, good old fashion push-ups. You could even try running silent rehersals if you need to.