One year before I became a junior high school student, the school I was planning to go to had a bullying incident.
ご質問ありがとうございます。
「一年前」はone year beforeとto the year beforeです。
「通う予定」はplanning to attendとplanning to go toに訳せてます。他の英訳もあります。
「いじめ」はbullyingと言います。
「〜が起きる」はXX happensとthere is a XX incidentとthere is a XX eventに訳せます。
ご参考になれば幸いです。
A year before I became a middle school student, a bullying incident happened at the middle school I was planning on attending.
"私が中学生になる1年前、通う予定の中学校でいじめが起きた"
- A year before I became a middle school student, a bullying incident happened at the middle school I was planning on attending.
- A bullying incident happened at the middle school I was planning on attending a year before I became a middle school student.
僕が将棋教室に通うようになる2年前、地震が起こった。
- Two years before I started attending shogi class, there was an earthquake.
- There was an earthquake two years before I started attending shogi class.
私が料理教室に通い始める三日前、友達と喧嘩した。
- Three days before I started going to cooking class, I had a fight with a friend.
- I had a fight with a friend three days before I started going top cooking class.
アメリカ合衆国 
