How can I repent and set things right with my parents after telling them about academic success that wasn't true, while fearing their possible reaction?

Lying is strictly forbidden, and it contradicts truthfulness, which is the foundation of faith. A true believer is not naturally inclined to lie. Therefore, you must repent to Allah from it and adhere to truthfulness. It would have been better for you not to lie to your father unless he would have suffered significant harm. You must complete your studies, and if your father asks you about your studies, you can resort to tawriyah (deliberately ambiguous speech) and allusions without resorting to lies.