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He sees them talking with a big smile, but they haven't got a clue.
Yeah, they're living the good life, can't see what he is going
'He says, "I'm talking with a big smile, but they haven't got a clue."' Meaning that he is depressed but he hides in by faking a smile. The popular kids don't see through that and the depressed boy would give anything to be a popular kid. It's a constant struggle.