* School years refer to the entire period of time a school is in session, typically spanning from fall to spring. It's a calendar-based timeframe. For example, the 2023-2024 school year.
* Grades refer to levels of schooling, usually numbered sequentially to indicate a student's academic progress. Grade 1, Grade 2, Grade 3, etc. are examples. A grade represents a specific stage of learning within a school year (or possibly across multiple school years in some systems).
In short, a school year is a time period, while a grade is a level of education within that time period (or sometimes spanning multiple periods). A student might progress through multiple grades during a single school year (in some cases, like summer school) or remain in the same grade over multiple school years (if they are repeating a grade).