Hi S. -
I just wanted to tell you that its totally up to you, but take the teachers suggestions into consideration b/c she is with him everyday and sees if he really has difficulty or not. My best friend's son was born at 28 weeks and he had to be held back in kindergarten. It was a very hard decision for them to make, and he was doing well in some aspects, but not others. They were more worried about him struggling to keep up and then feeling bad about himself when he just couldn't. The same thing you said, if you need to do it, then now is the time to do it.
Now my friend's son is in the 3rd grade and he is doing WONDERFUL! Keeping back that one year just did exactly what he needed, gave him one more year to catch up with everyone and learn the drill of school. He did fantastic his 2nd year of K and ever since.
I just thought I would share that with you b/c my friend had been through the exact same thing for the same reasons. Babies that are that preemie, certainly are not 'slow' per se, but in some cases it does just take them time to catch up on things, I really think its just age wise. She says that sometimes still he's a tiny bit behind his friends, but not noticeable for anyone else. It took him to about 6 years old and then he really caught up with everyone his age, with learning and his size and comprehension.
Best of luck in your decision!