Im pretty sure i have seen a lot of commitments as sophmores. Don't think you can sign until your a senior though. I really have never heard of an 8th grader commiting to a school before.
You probably can't offer but you can say when you're a junior we have a scholly waiting for you. You're not formally writing out an offer, but you are kind of offering one. I don't know how the system works but I'm sure it has to be written to be a legal offer.