The positive discipline idea-fine if the school doesn’t need different strokes for different folks. If it does, then simply don’t reward inappropriate behaviour, but given a token reward to every one else who behaves acceptably ie if they need to reach 500 points, 3 pts each.
Academic? well, if GCSE’s etc are as course & module-based as I believe, you award nil or a low mark where what’s presented is inadequate or missing homework.
If in-class behaviour in a certain subject is the problem, you exclude briefly, explaining to the kid that they are thus stuffing up their chances of passing every time it comes to this and the remedy is entirely in their hands.