This is the 2055th most frequent Chinese word.
中學
"中學" means "secondary school" or "middle school."
The term '中學' refers to 'middle school' (i.e., an educational institute for secondary schooling) in this sentence.
我弟弟今年開始上中學。
My younger brother started attending middle school this year.
Here '中學' is used to denote a specific period in a student's life, referring to the middle school years.
在中學期間,他學習了許多新技能。
During middle school, he learned many new skills.
In this case, '中學' points to the institution, the middle school, where the referenced teachers work.