I understand your line of thinking completely lucounu, no offense taken.
My suggestion was either/or (St. Johns would not accept CLEPS). CLEP out of as many classes at a regular college in order to focus on the more interesting ones or because Mia is only 15 and St. John's takes kids early, enjoy 4 years of outside the box thinking, being around true intellectual peers and stretching the brain. Then go to grad school and specialize. If she is like my DD (and it sounds like she is), she is either already or soon will be operating beyond the level taught at most 4 year colleges.
Critical thinking skills could only help her become a leader in any field she chose for grad school - just a thought.
