I'm not saying there are no negative effects. What I would say is this: even before the pandemic, many students were not sufficiently "challenged". They were already moving more slowly than they could have moved. It's possible for them to catch up. They could have caught up with other kids before the pandemic too, if put in the right situation.
Time spent sitting in seats is not education, by itself. How that time is used is just as important, and we often squander that time, even when there's not a public health crisis.