How do I proove what level my child is at if I want to put my homeschooled 15 year old into a public high school?

call the school and ask:)

Seriously, we are putting our 12 year old into public school next fall for a multitude of reasons the main one being he wants to give it a go. So, I called the school to find out what they needed. I was told that a simple standardized test for the grade we are finishing is a good gauge for placement for him along with a portfolio of his work and any other information. They also have some testing they do for actual class placement. They did not give me a hard time or seem like it would be difficult. So, I would call now to ask if you are thinking about fall so you have time to get your ducks in a row:) If you are looking for immediate placement, it is even more important to call the school to find what is needed.