Consider that in the late 90's cellphone penetration was still low enough that not being allowed cellphones was fairly common, and certainly not something anyone would react to.

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.