学年

1. Origins, structure, and basic meaning

• Structure: 学年 (がくねん) means a school grade or year-group. 二学年下 measures a difference; 学年ごとに classifies groups.
• Basic usage: A sister is two grades below the speaker; meeting classrooms differ for each year-group.

2. Common pitfalls and similar expressions

• Grade gap between people and grouping by school year
  • After a numeral it counts an academic-year difference; with ごとに it partitions students by grade. It is not simply a calendar year.