I feel like my answer is going to be so completely different, but it's a story I think needs to be shared. My son went to pre-school for 2 years and excelled. It was amazing! Then K came along. He was a bit nervous, but also excited. Enter: bullies. Yes, this starts in K. I caught on to it pretty early, and talked to the teacher. She did nothing. I wrote notes, called, and went to the school and nothing was done. I talked to the principal. He did nothing. I talked to the guidance counselor. She did nothing. My son had horrible regression (peeing in his pants b/c of all the stress) and the school didn't even notice or think maybe I should be called. For 7 months we tried to get the school to stop the bullying, as outlined in their bullying policy. It didn't happen. Then I emailed the director for elementary schools in our county. She finally was able to make things happen, but it was just some damage control. My son was dealing with it better, but they weren't stopping it. After he was still threatened, stolen from, and spit on on the bus, I pulled him out of that school and am homeschooling him. He's excelling now, with me, and the accidents have almost completely stopped. (He had to re-train his bladdar/body how to function. It was potty-training all over again 2 months before he turns 6.)
As far as academically, it helped my son some, but not that much. I was suprised b/c he knew how to print correctly and all of his letters before going in, and some 3 letter words, but they did not reinforce anything, so I had to spend the first week of homeschooling going over that. I learned that he was guessing words or remembering them (he has an amazing memory, like his daddy!) in the story. He was "reading" to me and he got every word right so I thought he knew them. He was just repeating though, not reading. Now we're working on phonics, etc again.
I don't want to scare you; most experiences I've heard about have been good. My son is in a school that is supposed to be good academically, but he has a really poor teacher and the principal is new so the school's record does not reflect him at all. He was also bullied on the bus as well and had a bad bus driver. How he was unlucky enough to get all three of those, I don't know, but that's what happened. Also, bullying seems to be a bad issue in that school. I've been in touch just this week with another mom who had the same thing happen to her daughter. I wish I'd known her sooner! She's been fighting the school system for 3 years now. But, what I do want is for you to be aware that this *can* happen. It doesn't mean it's likely, but it's possible. So before even going into this, find out the bullying policy. Get in touch with school board members and find out who to contact there if you have problems. Talk to your son about bullying-both being a bully and being a victim of bullying. Become educated yourself about problems like these and how to appropriately and effectively take care of them. Even if you never need to know, another mom you know might need that information some day. Also, talk to your spouse/significant other/etc about what to do if something like this happens to your child. Will you homeschool? Will you go to private? Will you transfer within the school system? There are options. After my experience, I don't trust anyone in that school system with my son, b/c in the end they wanted to blame him for being bullied. They said perhaps if he'd have gone to pre-school, then I told them he did for 2 years. They said maybe he wasn't used to boys. I told them all his best friends at church are boys, and they are a rough and tumble group and all the kids in pre-school were boys except for 2. They said he should have told a teacher, but she told him to just do it back. They said he should have told the bus driver but he said if my son didn't stop doing whatever (I never found out what-I was always told my son wasn't doing anything bad, which I knew wasn't right from a friend who subbed for his gym class), then he was just going to leave him with the principal. My poor son thought that meant he'd be taken away from his family permanently. He was VERY upset. To know all this, you have to have open communication with your child about it. Also, get a notebook for his stories from school. Anytime anything happens like bullying, write it down, along with the date, what you did, and what the school's response was. If I'd have done that, I'd have pulled him out long ago and wouldn't have kept trying to work with the school, but it's also proof that the school could use in punishing bullying.
Hopefully nothing bad will happen and it will be a great experience. But, if it does, it would be so much better/easier for you to make the best decision for your child if you have all the information I mentioned.
Good luck!