From the GCISD Course Selection Guide:

High school students may earn up to two (2.0) credits toward graduation through correspondence courses. Students wishing to enroll in correspondence courses MUST consult with a counselor prior to enrolling. Correspondence courses shall be used for emergencies or enrichment only and should not become a substitute for residence work. Courses are subject to approval by the principal. Students are responsible for paying all fees for course materials and resources. Correspondence courses are not included in the calculation of GPA or class rank. 

See below for correspondence course providers: TTUISD UT High School

Other Requirements:

If a student wishes to take Health via correspondence, the student must prove CPR certification prior to approval.
All correspondence courses must be COMPLETE before the first day of a student's senior year.

Students must submit the following online form to their school counselor PRIOR to enrolling in a correspondence course:

Required Approval Form