I changed schools in the middle of my school year at high school.
高校の学年の途中で転校しました。
おっしゃる通り、「～の途中で」はin the middle of 、「学年」はschool yearです。「～のせいで、～の理由で」というときは、because ofを使うといいですね。
例：
because of my father's job 父の仕事の関係で
"I often had to change schools in the middle of the school year because of my father's job."
- "I often had to change schools in the middle of the school year because of my father's job."
- 「父親の仕事の都合で、学年の途中でよく転校しました。」
関連する単語とフレーズ:
- **transfer schools** - 転校する
- 例: "I had to transfer schools several times."
- **move in the middle of the school year** - 学年の途中で引っ越す
- 例: "We had to move in the middle of the school year."
- **because of my father's job** - 父親の仕事の都合で
- 例: "We moved a lot because of my father's job."
日本 
アメリカ合衆国 
