I think there are some kids, maybe boys especially, who benefit from a strict regime, and perhaps they even need someone breathing down their neck 24/7. Some don't cope well with having lots of free time to manage themselves. Some really need the feeling of esprit de corps and some benefit from having something very concrete to kick against.
However, to imply that the problems of all the poor or underperforming are down to lack of discipline (as represented by military schools) is basically an attempt to present a narrative that blames impoverished and overstretched parents and underfunded schools for the dire academic underperformance figures, the ASBO rates, the lack of preparedness for employment of so many young people.
The real story is that parents live with the enormous stress that comes from working dead end, underpaid jobs and dealing with the welfare system as housing costs skyrocket and the owner class gets richer on their backs, schools that can't cater for the complex needs of children living in grinding poverty, and the very sorry state of both youth and adult mental health and addiction services.