That's the beauty of having a huge homeschooling community. By the time my kids are learning those things they will either be taking classes at community colleges where they can also learn languages, study art, chemistry, higher math etc. OR another option is professors that are in the homeschool community. There are many professionals and a lot of them teachers/professors that have classes & workshops.
A lot of kids learn online as well.
The other thing to keep in mind is you don't start with a high schooler obviously, you start with a kindergartener and you learn as they learn. When the girls and I start working on history I expect to learn a LOT that I have forgotten or missed the first time around

Same goes for other subjects. A great deal of parents learn with their kids and sometimes it is the kids teaching the parents which only reinforces what they learn
Our babysitter is 17 and is homeschooled. She will tell you her Mom was her primary teacher through 4th grade then she went mostly independant until she got to places where she needed help and then found it in the community. They hired an algebra tutor and used the community colleges.
What all the speakers that I have seen say is for the early years ages 4-7 you should only be spending about 1.5 hours on school. As they get a bit older, 3rd grade or so you can start spending 2-2.5 hours on school, 3 hours a day for middle school and 4-6 for high school depending on what they are studying.
There was a study done not too long ago and it found the average American high schooler only learns 1 hour 20 minutes in a full day of school. So much time is spent settling the class, reviewing what they went over yesterday, passing in homework, going over homework, talking about the next assignment and then of course there are interruptions and the whole social aspect of kids talking in class.
Most homeschoolers spend more time out of the home than in the home attending classes, workshops, zoos, museums, libraries, etc. There is a saying "If I'm a homeschool kid how come I'm never at home?"
LOL
PS I teach the girls together at this time. They each pick up on different things at a different pace. By the time Lilia is HSing I will be able to give KA and Avery busy work after some instruction when I am working with Lilia. Also, the girls will be great teachers for Lilia!