lougle - dd1's expereince of TEACCH was vastly different. BUt then, I don't think any school in the UK delivers TEACCH (which is part of the problem).
at dd1's (highly respected, much-lauded) ASD preschool (which I think is abysmal, and which has been described by more thanone professional I know as the worst example of an ASD preschool they ahve ever come across!), TEACCH was delivered thus:
individual workstatins, all arranged around the edge of the room (facing the wall, so dc all had backs to the room). a series of 'tasks' presented on trays (various tasks, form matching to sorting, to latch boards - anythign that could be stuck on a palstic tray!) - trays lined up on the left hand side of workstation. to be taken individually (by the child, no adult mediation), completed, then passed ot the right. the child had no interaction at all, was jsut expected (when it was 'worktime' - I must stress not all her time was psent at the workstations!) to sit down, and get on with it. apparently, they would be able to work out the tasks involved for each tray individually, and get on with it individually, with no adult involvement.
the reward for completing wa sot be able to get down formt he workstation. and yes, dd1 was held into the chair more than once - she did not find any part of it motivating or rewarding (dd1 is all about social involvement), so hated sitting facing away form everyone. her headteacher once described ot me, with amusement, how she would get a dc sitting, then casually lean on the back of their chair, so that they couldn't jsut get up. I didn't find it so funny 
It does all come down to: good school and good staff (I would love to employ zzzzz as a tutor for my lot, or as a nanny or similar - the enthusiasm and utterly bonkersly fabulous ideas she comes up with are amazing) = everything working. crappy school and shocking staff = hideous experience.
the difference (from the little bit you describe) between your dd1's school and my dd1's school seems ot be the level of staff (dd1 needs 1:1 to keep her attention) and the level of personalisation of the rewards (dd1 wouldn't understand or get the 'specialness' of a class treasure box - so her rewards for the session are all chosen each morning, by the tutors who will be working with ehr (different tutors=different reinforcement, as a different relationship with each). and that's it. again, no mystery to ABA