One of the virtues of employment law is that it leads to an improvement in employment standards: the general standards of employee treatment in offices, shops and factories are far better now than they were 30 years ago, and this is partly attributable to the desire of employers to avoid tribunal claims. Flying schools appear to lie far outside the norm on this. One or more of you junior instructors really needs to send a message out by suing one of these outfits. The working time and minimum wage regulations might afford a means of doing this. FFF, note also that a deduction from pay is generally unlawful if not agreed in writing.
People may reply "We won't get another job if we sue a flying school", but that attitude helps to perpetuate the abuses. AOPA could earn some brownie points by campaigning on this issue and/or by funding a case or two. Or maybe join the T & G.