K.C.
I worry about the same thing. My husband and I are from countries that start kindergarten at age three or four, depending on the area and the child's abilities. Our 14-month-old already knows some letters and numbers. There's no way we're waiting until she's 5 to teach her to read and write, but here, school doesn't start until 5.
Our only option seems to be homeschooling since we can't afford private school and are ethically against it anyway.
Luckily, homeschooling isn't about social isolation. There are associations and groups so your child can meet up with other homeschooled kids. I recently signed up for a group here: http://groups.yahoo.com/group/LVLifelearners/ that has homeschoolers who can give lots more advice.
You can also do a Google search in your area to find homeschool associations. They tend to fall into two groups from my preliminary research: people who do it for religious reasons and people who do it because they don't trust the schools. Both groups seem to get along despite political differences.