Advocating for proper placement can be extremely frustrating, but it might help to try to think through how this may all appear from the principal and school staff's point of view. This is just my experience, so take it fwiw (not much lol!)... also please know I'm not saying this is true of your child, but instead I suspect something like this may be shaping the principal's reaction to your request. When my children were in preschool and K-1 it seemed like we were surrounded by parents who were convinced their children were brilliantly gifted, quite a few of them had children who were reading while still in preschool, and most of them were pushing for gifted program services at school when they entered K. By the time my kids were in 2nd grade, most of those parents no longer mentioned anything about "gifted", there children were no longer far ahead of the bell curve in reading, and ultimately very few of them made it into the gifted program in our school district. It doesn't mean they aren't bright kids, and it doesn't mean that gifted kids somehow "level off" in school - it simply means that the school is most likely hearing from quite a few parents that their just-starting school K-1 kid is unusually gifted and needs support - and that ultimately, many of those kids probably fall out in the top 80% of the class, but not the top 95-99th % of the class. I think you may also potentially run into a bit of bias against home-schooling, as I've seen that in some of my friends who are educators. Not all educators have that bias, and I've known *many* amazing kids who were home-schooled - but it's a possibility.
Another thought - let's say that the principal fully believes that your ds has mastered all the 2nd grade curriculum while at home, he might still be resistant to skip him because he feels that he's perhaps mastered it under unusual circumstances and therefore isn't convinced he'd continue with the same progress once back in the classroom. He might also believe that under the same circumstances, many of his other students could also advance faster than the curriculum allows. Perhaps he believes that at his school classes are already working ahead of grade level or differentiation is good.
Given all of that, I second everything Cricket2 said - hard data is difficult to argue. I would also put future communication re testing etc to the school in writing (email is ok).
Hang in there and keep advocating!
polarbear
