Sixteen for zero!!!
Intrinsic and extrinsic motivation! If the other students are concentrating, and thereby improving, you are in a very difficult spot: you could work yourself to death by assuming that the problem is yours. The point here is how much energy you are prepared to invest in this group. You could completely restructure the classes, providing an entire ‘submodality’ that provides hyper-interesting and broken-down-into-smaller-chunks versions of the work that the rest of the students are doing – see Working Yourself to Death above. You could put them at the front of the class and ask them questions, as you go, about the work – old-fashioned, but effective.
The fact that the parents are genuinely interested seems to be your best point of leverage. I would speak individually to the students first, and then as a group. Explain to them that the choice is theirs: you have a responsibility to their parents and your employer to teach them. If they won’t respond to this approach, that they have a chance to right the problem themselves, then you could arrange a meeting with both the parents and the kids, state the problem, list the solutions (a separate, paid, class for the group? extra homework?), and see what happens.
In closing, if you are in this profession for the long haul, understand that this is the nature of the beast: some students don’t care – and sorry, ‘lack of concentration’ is not caring -- and you can hold your breath until you go blue in the face and it won’t help. Sometimes, sadly, sadly, sadly, it’s a matter of the greatest good for the greatest number.