Standing up in front of the class to deliver a speech or recite a poem is what we used to do in the 'good old days'. Don't see any reason why that couldn't be adapted for homeschoolers. First recite in front of the family, then your homeschooling activity group, then a church group, and so on. Work up to it gradually!
Toastmasters is a good training ground for grownups. I don't know if they'd let high school age homeschoolers in, but it would be worth asking.
Giving presentations and demonstrations in 4-H in front of judges works pretty well also.
If I'm not mistaken, I believe there is some sort of 'junior' Toastmasters group.