I don't think the point is whether the schools require a high school diploma as a stated prerequisite; it's more whether they actually admit people with no diploma on the basis of other work.
In cases where diplomas are required, the acceptance letter will always contain language stating that they will retract your acceptance if your grades drop an unacceptable amount or you don't finish the degree you're working on. Then most schools require you to submit another set of transcripts, proof of graduation, etc. before you officially enroll.
I mean, didn't you ever hear of Doogie Howser?