How many tune out?

We've started first grade with Madeleine. Since we didn't take three months off for her to forget what she's learned, we rocketed through about the first quarter of reading and spelling. Phonics is easy and so we're into the second quarter of that, too. First grade math we started back in January, I think, and science was easy to catch up on.
We're doing stuff now, in almost-May of her "kindergarten" year, that if she were in school she'd be covering after Halloween, maybe Thanksgiving of first grade.
I'll admit, though, it's taken her upwards of 45 minutes to do her two-page math assignment some days. Not because it's difficult but because it doesn't hold her interest. It was too easy.
This isn't about how terrific and bright Madeleine is (even though I'll admit I believe that to be true). It's not about how bored academically I think she'd be in a regular school setting.

It's about tuning out. I think she'd do what a lot of kids do, what I think I may have done. I think she would tune out the teacher as she explained place value for the third day in a row, the same as she'd quit listening as the teacher drilled them on the sounds of D, L, F, or M for the fifth time.
I wonder how many other kids tune out the teacher and forget to tune back in, or decide subconsciously that they can get B's and C's paying attention a quarter of the time and it's not worth it to do more.
When I'm a foot and a half away, sitting at the same table, it's impossible to tune out completely or for very long. Not when the teacher is paying that close of attention, that is. Besides, she can adjust the curriculum to be a tad more challenging and thus interesting.
It just means that every day I find another reason that homeschooling was the right decision for us.