If a high school wanted to lead a Christan study group and wanted a free lesson plan, where would you go?

You will have to do this outside of school. It's illegal to do it in school plus school is not the place for worshipping - you have plenty of time to that out of school there is barely enough school time to learn what you need to learn. Do this group on your own time, preferably at a church that's what they are there for!