I'm so grateful for the above posters and comments. I'm going through a similar situation with my eg/pg son (6 yr) in kindergarten.

My son's been in two private gifted schools (both aimed at HG/MG) since last year and we're about to pull him from the second one and homeschool him. We tried a number of times working with both schools, but we've found the teachers/principals inflexible about grade skipping and other policies. Then again, we also ran into social/emotional issues with both schools in terms of my son and other children.

So I agree that a lot of revolves around the teachers/administration as well as the other kids. It is case of Russian Roulette.

I've found other issues too crop up with having a eg/pg who's 3-5 years ahead of his age and peers.
1. Confidence/output. Children do not always want their ideas/thinking/abilities to stand out. My son read two books at a 4th grade level (Charlie and the Chocolate Factory and the sequel) but no one else at the gifted school is reading 4-5 levels ahead. My son doesn't want to stand out so now he's reading at a 2/3 grade level again.
2. Acceptance/tolerance. Some pg kids can be in an academic setting who are less gifted, but others will not tolerate it long-term. We had this happened with our son, especially at the 1st gifted school. He whizzed through the pre-k/k/1st grade curriculum in 2 1/2 months and then started to act up because he was bored and wanted to be out asap. At both schools, we've found that our son has trouble relating to kids his age and vice versa. This has contributed to problems. He's done better at the second gifted school in a mixed-aged setting, but we've also come across older kids who don't know what to make of him, not mature, or 'resent' him (for lack of a better word).
3. Social/emotional. Your son may be socially/emotionally more mature than other kids his age, but others may not be as advanced. Some kids are fine with this, but other kids like my son are less tolerant.
4. Curriculum. Teacher/child-driven. We found our son totally resisted the drill-kill, rote learning, teacher-driven approach at the first gifted school. Total disaster. At the second gifted school, it's been child-driven/ intrinsically driven and there's been more success. Still, my son is miserable with the way the teachers at the second gifted school approach math, especially the timed test method. He's been less thrilled with some of the overemphasis on arts/crafts and experiential learning than on content. Upon reflection, I'd say you need to strike a happy balance between content and extras and intrinsic/external motivation to learning.

If the curriculum isn't rigorous enough or enough depth/content for your pg (like we've found), then it can be a difficult bridge to cross if your son isn't getting enough to sustain him at a school setting. And of course, there are other curriculum issues that pg parents face, which some of the above posters have mentioned. Even if you're child is reading only 3 grade levels ahead, you still have to monitor what they're reading in terms of social/emotional issues. I forgot how much authors write about bullying, teasing, girl/boy stuff, sibling rivalry, etc. in books aimed at typical 3rd grade children. It starts early and yet your child may or may not be ready.

PG kids tend to do better in mixed-age school settings so they do not stand out, but again it depends on the child, teachers, school, and the other students.