One thing i've seen work well and used once (but not enough) is a points system. Trying to focus on the positives as wellas giving sanctions. The way I tried it was they got points for things like everyone standing behind their chair, everyone writing down the objective, everyone listening when i'm giving instructions, stuff like that. Write them up on the board so you can rub them off again if they're naughty. THen if they, as a class, got 5 points in the lesson they earned a lesson in the ICT room. Worked well with my wayward yr 8s as they started to get annoyed with each other for getting points knocked off rather than getting annoyed at me for singling them out as naughty ones. It actually took them 2 lessons to get to 5 points but i let them go to ICT anyway. You could perhaps adapt this - with permission from your H.O.D. - and tell the class that if they get 5 points for good behaviour in a theory lesson, or over a series of lessons, they can do practical next time..?