Nope, the IHSAA does not allowed co-ops
There is an exception for individual students based on curriculum
"Q. 12-8 If a student lives in the attendance area of Public School A, but Enrolls at Public School B (a School which offers innovative teaching methods and has no athletic programs involving an IHSAA Recognized Sport), can the student play football on School A’s football team?
A. Yes, provided (i) Public School A accepts the student in its football program, (ii) the student, Public School A and Public School B, each inform the IHSAA of student’s participation in Public School A’s football program, (iii) there exist no apparent IHSAA rule violations and (iv) the student meets all IHSAA eligibility standards and all of Public School B’s standards. (rule 12-4)"