How do you teach something you don't know? You learn it. Alternatively, you could get help.
I followed a link from an article about how a quarter of all teachers have fewer than 5 years experience to the comments about the original article. One person said that her problem with homeschooling is that her son already knows more than she does about things like calculus and computer science. "How could I teach these skills to my son?" she asks. "He needs to be taught by experts."
First, your school may not offer the classes he needs for math or technology. I attended a smallish high school and some of the more advanced stuff was simply not available.
Second, you're not guaranteed to learn from an expert. My teacher friends have been thrown into situations where they have to teach a class they are not prepared to teach. They are forced to scramble and create lesson plans and whatnot. Having been on the instruction side of such a class, it was clear the teacher was not an expert.
Third, as a homeschooler, you have access to some of the greatest experts out there! You can get similar (if not superior) texts and tools that teachers use. In fact, many advanced courses can be purchased where an expert walks you and your student through the material. I'd gladly take a published course over a first time teacher's bumbling attempts.
Forth, this argument often assumes that you are jumping in with your student without any background. But if you have tackled arithmetic, geometry, algebra, and trigonometry... calculus would be totally do-able! As a homeschool parent, you get to learn alongside your children. And that means you are often prepared enough to help them master the material.
If your student wants to learn something you don't know, finding an expert may be just the thing. You could order a video or online course, find something for free through YouTube or iTunes, get a knowledgeable friend to help out, or enroll your student in a local class. ...and that's truly the beauty of homeschooling: We are not bound to only learn at home. We can take advantage of any and every option that is best for our family.
As something of an "expert" in the field of teaching movie making, I've had four teachers contact me about my free film school. They have been tasked by their school to create a film class, and reach out to the "expert" -- me -- for help. Because the expert teachers know that they can learn things they don't know to teach their students something new. You can too.
If there's a subject your student is interested in, Sonlight just may carry an excellent resource for you.
Filmmaker, Writer, Guardian