That assumes that behaviour, disruption, etc in schools hasn't changed.
In reality, it's changed massively for the worse.
When schools were relatively calm, controlled, with generally well behaved children, the shy, anxious, stressed, ND, children could mostly function. Even moreso when a lot of the education was done on single desks, individual "quiet" work, etc.
Now many schools are more like a warzone with out of control kids, loud, shouty, disruptive, etc., the anxious/ND/shy/stressed kids find it harder to cope. Added into that, most classrooms now have clusters of desks or pupils sat around a big table, plus ever more groupwork, meaning less opportunities for pupils who prefer to study quietly on their own.
The ND and other "special" needs kids have always been there, it's just that previously schools were better managed, kids were better controlled, etc., so they could cope. It's not the struggling kids who have changed, it's the entire environment in which they are forced to work within that's changed.