I think by MIT high school, Alexz825Mom meant a strong Math/Science school.
TJHSST is a public (or selective enrollment) high school in Virginia, I believe. For both IMSA and TJHSST my opinion is kind of "If you haven't heard about it, or aren't an adult with small children or no kids at all, don't worry about them."
If you live in Illinois, and you're passionate about the sciences, definitely seriously consider it. If you're going into ninth grade, still apply. Don't be too upset if you don't get in as a rising Freshman.
I never seriously considered IMSA, as I am not a Math/Science person. Illinois History and Philosophy Academy would be more my school, if only it existed. *wistful*