"Nothing"

If you have any experience of teenagers, you'll be very familiar with the response of "nothing" when you ask them "What did you do at school today?"

They're not being sullen, uncommunicative, or rude.
They're not 'so addicted to their screens they've forgotten how to have a conversation with real people!'  
They're not keeping secrets.

What did you do at work today?
What did you do when you went out earlier?
What did you do at the weekend?

I suspect a significant number of you, at least initially, thought "nothing" or "nothing much".

Clearly, 'nothing' doesn't actually mean nothing, either when we say it as adults, or when children say it.

A proof of this: Ask a five year old what they did at school - but make sure you have at least an hour to listen to the answer!

Ask a teenager, without judgement, what happens in a gaming stream, or what's going on in the book they're reading, or how their latest art project is going - you'll almost certainly find yourself in a very interesting, perhaps quite intense, but wholly adult conversation.

'Nothing' most often means "nothing that matters."

And we can engage with that. Not by demanding to know 'why you're doing things that don't matter!' (in the case of teenagers and school, because they're legally obliged to.)  Not by sneering that 'it's down to you to make things matter, you know!', but simply by accepting the fact that this individual is not currently engaged in something which offers them any sense of meaning, or which they feel will take them anywhere in their life.

"That's a shame - what would you rather have been doing?"
The answer which may come first might be "Dunno."
This, again, is an opportunity; make time, in an unobtrusive way, to come alongside that person while they are enjoying something. Listen to them while they are talking about something they're passionate about. (Yes, even if it's 10pm, and you just want to sit down with the News and begin to unwind from the day. Yes, even if it's 8am on a Sunday morning, and you want a lie-in.) 
Over time, you'll come to understand what's important to that person - what matters to them.

And then you can start exploring how to resolve the fact that they're spending a considerable amount of their time doing something that doesn't matter to them.

Beyond learning how to be curious about people who aren't us, how to 'read the room' so you know what energy to bring (and how to identify what energy you're bringing, and know how to alter it), how to read, write, follow and conduct an argument centred on the issues, rather than the personalities involved, and how to carry out basic maths, is there really any need for children to be in school if it doesn't matter to them?

"But I wouldn't be able to go to work if my kid wasn't in school!"

Wouldn't you? Does 'work' have to be something that mostly benefits someone else? Couldn't you bring your children in on creating a business that would benefit you, and them? If we removed the obligation for children to attend school once they'd acquired the functional knowledge we've discussed, might we not see a wide variety of childcare settings, focused on different skills, personalities, and interests - meaning you could still go to work?

Start with 'nothing'.
Accept it as a complete sentence, and a statement full of meaning.

Let me know where you end up from there.