I had a similar situation when my ds was 3yo. I wasn't really so much concerned with them challenging him as much as I WAS concerned about:
1) them not squashing his love of learning by forcing him to do work that was below his level simply as a means of teaching him how to follow directions (with the mindset that: because he already knew how to do those things, the frustration level would be with following the directions vs. following directions AND completing the task)
2) their ability to direct his interpersonal development. By this, I really meant: are they truly going to be able to supervise and mentor his relational skills in that setting better than I could do as a homeschooling mother? Who's going to be responsible for this in the classroom? A teacher with training specific to my kid's needs? Or the aide assigned to him who has a high school diploma and MAYBE some specialized training (if at all)?
It really came down to seeing the classroom in session first-hand. I'll be honest: I ADORED the preschool disabled teacher my son would've been with. She believed in all the same literature that I did--had studied much of the theories I was most supportive of. She was really good with the kids.
But at the end of the day, the classroom setting wasn't going to be a good fit for my son. At the time, he had more oppositional behavior than he does now. In a school setting, there was absolutely no way they would legally be able to work him through that in ways that were going to produce the results that I wanted for my child. They wouldn't be allowed to hug and rock him until he calmed down from a rage. They wouldn't know him well enough to understand when it was okay for him to be upset for any length of time (at least not till mid-year). These were very big deals. And with no real way to redirect him into compliance (he had no real "currency" for praise and frankly, I'm not "for" the whole "If you do this, you can do/have that" mentality anyway) I just felt that it was a recipe for him seeing the classroom as a place of no real consequences for poor behavior... and that becoming the mentality about school.
Instead, we tried two different private schools--moving mid-year when the first didn't work out. Neither was a good fit.
The following year (what would've been his pre-K year) we kept him home. I dreaded it given his behavior issues; but ultimately it was the best thing ever. I was able to supervise and mentor his relational skills in a way that made so much more progress. Of course, I also made a serious effort to have him out and about a lot. About halfway through that year, the behavior started to level off and things were really good.
He just turned 6yo and would be in Kindy, but he's home and really doing so much better than I could've imagined. His PDD-NOS dx was lifted in the fall and we are finishing up our services with the district (I take him to the school for this 4x/week for 30mins each--currently just OT) and then we're done.
For US, it was the best decision. The people that gave me crap about him needing the socialization are baffled when I explain that children like my son truly needed more help with socialization--help that *I* could give him by being there all the time. I didn't intervene, but I was there. I saw what happened first-hand vs. what the teacher managed to catch and/or he remembered from the day. I was able to work him through it right after it happened while it was fresh in his mind (and heart). Sometimes I was able to work him through it while it happened (although this was rare). And I was always modeling behaviors for him... when we were at stores, the library, at group activities. He really couldn't have had better social skills training and he couldn't have had it more intensively. Furthermore, it was social skills training that I knew I was in line with, because *I* was giving it to him.
Again, this was what was best for MY kid. And he WAS in a classroom setting until his pre-k year. But my child was becoming the "problem child" and THAT was also something I didn't want to stick.
I'm not sure this helps you at all... but there were no responses, so I thought I'd share.