Parents had to stay in the gymnasium/sports hall after the entrance ceremony to decide board members, but we couldn't decide after all.
Parents had to stay in the gymnasium/sports hall after the entrance ceremony to decide board members, but we couldn't decide after all.
（保護者は入学式の後も、役員を決めるために体育館に残らないとならなかったが、結局決められなかった。）
「残された」の部分は「残らなければならなかった」としてParents had to stayとしました。
体育館はgymnasiumまたはsports hall、入学式はentrance ceremonyです。
役員はboard membersで、PTAの役員ならそのまま PTA board membersとなります。
参考になれば幸いです。
The parents were kept in the gym for the committee election, but it wasn’t decided.
**The parents were kept in the gym for the committee election, but it wasn’t decided.**
**After the entrance ceremony, some roles were filled by volunteers, but the rest remained undecided.**
**1. The parents were kept in the gym**
「体育館に残された」は直訳すると "were left in the gym" でも良いですが、意図的に“留め置かれた”ニュアンスを出すなら "were kept" や "had to stay" を使うと自然です。
**2. for the committee election**
“役員決め” は "committee election" や "parent committee selection" という表現が使えます。
日本 
アメリカ合衆国 
