That's a difficult question. One of our boys was in much the same situation. We did not hold him back and he has matured. He's now in 2nd grade and you wouldn't know he was so far behind socially just two years ago. Being around typically developing peers often helps.

I think the most important person will be the role of the teacher to help them mix around, or whenever there is a need to intervene to teach the mild autistic kid the correct way to communicate with peers.

If the class teacher looks capable of doing that, I would strongly encourage for the mild autistic kid to be placed in mainstream class.

Many factors - including dev level, how far from age cut off, and size, among others - but I'd lean strongly toward yes. Its an easy time to give an extra year wo child feeling like a failure which gets nearly impossible after K so much easier to regret not dong it. An extra year may put him much more on level for good relationships with classroom peers. My daughter is oldest in class and I am so glad,

If he/she is just mildly autistic, I wouldn't hold them back. He/she will learn from their peers and may flourish!