For us, there wasn't any real "tipping" point, so much as a major epiphany.
I don't know why, but even after we struggled for six months to find a preschool (and eventually gave up, opting for a very loose Montessori at home approach), we still were 100% convinced that we'd be enrolling our then 5yo in a local public kindergarten.
Our local school is a good one, within walking distance of our home. Both my DH and I are products of public schools. My family is filled with public educators. I believe in that model. I do not believe in unschooling as a model of education except for some very remarkable circumstances.
So our epiphany occurred when I took my then newly-turned 5yo to kindergarten orientation. I knew that I was going to need to talk to administrators/teachers and find out the lay of the land, so to speak, on managing her life-threatening food allergies (let's just say that most people who THINK that they know about life-threatening food allergies, even, haven't encountered anyone like her, and this includes a number of physicians-- she's nobody's "starter" project)...
my daughter was quietly content to observe all that was going on around her, politely sitting and listening to the presentation (geared toward parents) re: kindergarten readiness skills that parents should "work on" over the summer... all of which she knew. But that wasn't why we were there. We were there to figure out how big an uphill battle it was going to be to get her included in classroom activities and not, you know, kill her.
So during the mix/mingle afterwards, she sat and continued reading while I sought information from anyone official-looking.
I explained her situation to a veteran teacher, who agreed with me that {local school} was an excellent one, isn't that great... she described the handling of food within kindergarten classrooms and I did sort of quail, I fear... then she informed me that there were NO school nurses in any building in our district, oh yes, and the cafeteria (which doubles as the gym) serves PB+J as a menu option every.school.day.of.the.year. (OMG)... at which point, I was reeling on that front, given how far we'd have to go to make it feasible...
so then I added DD's concern from the earlier presentation-- what do you do with children who are already possess quite good literacy and numeracy as entering 5yo's? She had clearly heard that question before. She said-- "Oh, there are always a few that have already started reading because their parents have worked on those skills. Teachers are used to seeing quite a range in kindergarten. Did your daughter come with you tonight?"
I responded, "Yes, that's her over there against the back wall-- she's the blonde little girl reading." You have to imagine the scene, here-- there are adults milling around talking, and about seventy 4-7yo children playing tag, shrieking, and generally burning off sitting for forty minutes through this presentation at the beginning of things. DD was clearly aware of all of that, but was reading a very fat chapter book, swinging her legs gently back and forth and rapidly turning pages every thirty seconds or so.
The teacher took all of this in, and then turned back to me...
"What... is... she... reading?"
I just shrugged-- she read all the time-- I seriously wasn't even paying that much attention by this time. Hank the Cowdog, I think? Maybe Magic Treehouse? She went through them like potato chips. She read everything basically, and had only learned to read about three months ago, so I wasn't really sure...
"Does she do math like that, too?"
I sort of casually explained what I thought were pretty straightforward skills, nothing too special... but the teacher's eyes bugged.
"I have never said this to anyone, and if you repeat it, I'll deny it. DO NOT put your daughter into a kindergarten classroom in this district. You'll be risking her life every day, and she will be learning NOTHING. Don't do it. I'm so sorry."
I was stunned.
When I called the principal of the local school later in the week, though-- the response kind of chilled me, because that was a cheery "Oh, don't worry... we'll figure out the food thing as we go. Just understand that we're all going to make a few mistakes as we figure things out, and all will be well..."
Well, that was that. My DH and I both knew what that meant. That meant that they didn't have experience with this kind of sensitivity/severity, but THOUGHT that it wouldn't be a big deal. We knew that "mistakes" cannot happen if they can be foreseen-- every 'mistake' is rolling the roulette wheel, and the odds are similar (according to our medical professionals). Our original allergist here had also recommended AGAINST a school placement for DD way back when she was just three.
After that orientation, I started seriously researching homeschooling philosophies and methodology. We were still thinking that we'd eventually put her into a public school placement when she was "old enough to manage" on her own-- maybe 8-10yo?
The trouble was that after 18 months of homeschooling, she was at 3rd grade level (and then some-- actually, and then quite a lot in some areas) and climbing rapidly. That was when the virtual school came into things. We were having an increasingly difficult time getting her into appropriate extracurriculars given the huge gap between her chronological age and her cognitive needs, and around here, people who work with youth all have learned to roll their eyes at the exaggerations of mamas who proclaim their children future Einsteins. (Note that teacher's initial response to me-- before she was presented with the reality-- we've had this happen many times here.)
