We are homeschoolers, so all the registration stuff came to me. I received a registration brochure for the 10 along with our test results. I believe that anyone can take the 10, though they promote it to the kids who do well on the 8. You should be able to find all the information you need on the AMC web site. I'm not sure how it works if the math teacher is clueless- maybe you can meet with him/her and make your plans.

good luck!