Fix Bugs in Grade System

I have a compact student-grade management script that runs end-to-end without crashing, yet two key functions are silently wrong. After a recent refactor, both • get_student_grades() • calculate_average() now return the wrong data type, surface unexpected values, or give no output at all... (Budget: $10 - $30 USD, Jobs: Algorithm, Debugging, Documentation, Performance Tuning, Programming, Python, Scripting, Software Development, Software Engineering, Software Testing)