I understand your frustration. We had a very similar situation with our son. What made the enormous difference for our son was sessions with an Educational Therapist. Our son knew the information, but when he sat down for the test, his anxiety caused him to block the information and he would fail consistently. The therapist taught him how to relax, develop positive affirmation statements, breathe slowly and deeply before any test (similar to yoga breathing). He would consistently perform these behaviors before all of his tests. His senior year, he was taking three hard AP classes (Calculus, Chemistry, and Computer Science). He earned straight A’s in Semester 1 and almost straight A’s the second semester (one B). He’s in his freshman year in college and got an A in both Calculus II and Chem. Targeting the test anxiety directly was the key for him. Good luck.