Texas Tech University, via a kind reader’s email:

TTUISD has a comprehensive curriculum – coursework is offered in all required subject areas from kindergarten through high school. Our Credit by Examinations (CBEs) allow students to test out of subjects.
TTUISD offers all courses required for a high school diploma in the state of Texas. Our elementary level lesson plans require no prior teaching experience to use, and all TTUISD courses are written by Texas-certified teachers.
TTUISD high school students may choose between a Minimum Graduation Plan and a Recommended Graduation Plan (College Preparatory Program). Students who successfully complete high school requirements and pass the exit-level TAKS will earn an accredited Texas high school diploma.
Students must take a minimum of four full courses to be considered a full-time TTUISD student. CBEs do not count when determining full-time student status.

Perhaps nearby UW-Madison will give K-12 another try?