How can one make up for what was missed of the Sharia knowledge that was studied in school without understanding or application? And will a person be held accountable for what they learned but did not apply, despite being compelled to go to school?

A person is accountable for neglecting scientific and practical obligations, and for committing unlawful acts. They must repent to Allah Almighty by obeying His commands and refraining from His prohibitions, regretting their shortcomings, and resolving not to return to disobedience. A person is not sinful for neglecting virtues and recommended acts. It is obligatory to strive to learn what must be learned regarding the rulings of creed, prayer, and all other individual obligations.