It has been decided that each/every class will have one day off each week.
It has been decided that each/every class will have one vacation day per week.
ご質問ありがとうございます・
「1週間に1回」は1 per weekとか1 each weekとか１ every weekなどになります。
「休みの日」はday offとかvacation dayとかrest dayなどになりますが、学校の話ではday offとvacation dayがよく使っています。
例文：It was decided that each class will have a day off every week.
ご参考になれば幸いです。
Every class has decided which day of the week to take off.
"どのクラスも１週間に１回休みの日が決められました"
- The day for each class to take off has been decided.
- It has been decided for what day of the week each class will not be in session.
説明を読んだあと：
「どのクラスも１週間に１回休みの日が決められました。休みの曜日はクラスごとに違います。」
- It has been decided that each class will have one day off each week. The day off will differ between classes.
Due to a lack of teachers, it has been decided that each class will have one day off each week.
ご質問ありがとうございます。
「（先生不足のため）どのクラスも１週間に１回休みの日が決められました」は英語で「Due to a lack of teachers, it has been decided that each class will have one day off each week.」と言えます。
ご参考になれば幸いです。
アメリカ合衆国 
