That's because I really want the teacher's lessons to be fully attended.
That's because I really want the teacher's lessons to be filled.
For a hard-working teacher like that I think it would be great for them to have a full class.
eririnさん、こんばんは。
いくつかの言い方がありますか、その中上記の英訳例を述べました。
「満員」は "fully attended", "filled" , "full capacity" , "filled to capacity" , "sold out" (売り切れのニュアンス）,"full class" などで英訳できます。
例文〜
「頑張っている先生のレッスンが満員になることは良いことだと思うから」＝＞
"For a hard-working teacher like that I think it would be great for them to have a full class."
ご参考になれば幸いです。
アメリカ合衆国 
