A lot of times, my parents (mother and stepfather) played it off as "he needs to be disciplined etc", although they omitted certain truths when it came time to talk to the police. A punch to the side of the head was a "clip over the ear" etc. The police however seemed not to care, I will never forget hearing their "Ah yeah." reactions to it, but again they face the same thing as mentioned above. Even if you're not legally allowed to hit a child now, it's still seen in some places here as an unwritten omission of law to discipline your child if it is "Reasonable", such as a smack on the bum etc.
Indeed being kept inside the home wasn't really the bad part of it all. I have always been much of a loner, and it was very likely I wouldn't have done much outside of the home. I did sometimes "Run away" for nights, and I would walk through the city or walk out into the country for a very very very long time at night wondering where to go or what to do. Now that I look back on it, I'm actually very surprised albeit greatful that I never was abducted or robbed. But at that point, I didn't really care where I went or what happened because anything seemed better than what was going on at home.
Not being allowed to leave the house after school hours, or use the internet is still a fairly common situation for kids in many sects that are not considered problematic enough to interfere with.
What you describe as "being kept captive", others would consider protecting their children from corrupting outside influences (I'd like to emphasise that I absolutely don't think that is right).
That the stepfather in this case seems to have been motivated by hiding from the authorities doesn't make much difference - the point is most schools have students that are in similar situations, so it's not something most schools would see as exceptional and something they can interfere with.
It's a difficult tradeoff in terms of child protection, because the traumatic experiences of being taken into care or disrupted by lots of interference can easily become worse than what the interference is meant to fix, and because the moment you start passing moral judgement over peoples parenting, you're in for a crazy struggle over how it is acceptable for people to raise their kids.