What does it mean to maintain a yearly average in school?

Maintaining a yearly average in school means keeping your grades at a consistent level throughout the entire academic year so that your final average at the end of the year falls within a certain acceptable range (e.g., a "B" average or above). It doesn't mean your grade is the same in every single class or every single marking period, but rather that your overall average across all your classes stays relatively stable and meets the desired level.