Ugh before I was always reading on how unsocial kids are that "homeschool" now I am reading a lot about "Well what makes a parent think they can teach all subjects all the way through high school?!" What they fail to see is that most teachers aren't even teaching the classes they majored in. A teacher that teaches English might be teaching a math class and so on. I know this because I happen to know a handful of teachers that have told me this.
Not only that but a teacher is also taught how to manage and teach a group of students. A homeschool parent only has to teach as many kids as they have. Even if a parent had twelve kids that still wouldn't be the average class size. Sure there are some schools that have fewer students in class but that's not the norm.
Now homeschooling parents are also not the only teachers in their children's lives. Some are in homeschool co-ops where different subjects might be taught by a different homeschool parent that is more knowledgeable in that subject. Other's have family members and friends that may help. For instance my boy's wanted to learn some sign language. Now I do not know much of any sign language but our neighbor is and she is only too happy to teach the boy's some signs. I do not have to know all subjects I just have to know where I can get help from.
Also you have to remember teenagers can and do take some collage courses to learn what they need to know. There has been as young as 16 year old's taking collage courses for things their parents didn't feel they could teach.
Since my children are young I do a lot of learning with them. What I don't know we find a way to find out what we want to know about. There is so many resource's out there for homeschooler's so a teaching credential's we really don't need. I really wish people that decide to criticize homeschooler's would first do their research and then come up of a real con of homeschooling. Sorry I know I am ranting but it really gets old after a while.