The last time we had our lesson, you were already working there.
You already had your job at the call center when we had our last lesson.
ーThe last time we had our lesson, you were already working there.
「前回レッスンをした時、もうそこで働いていたんだね」
the last time .... で「前回…した時」と言えます。
to work there で「そこで働く」
ーYou already had your job at the call center when we had our last lesson.
「前回のレッスンの時、あなたはもうコールセンターでの仕事をしてたんだ」
to have one's job at ... で「…で仕事がある・…で仕事をする」
ご参考まで！
カナダ 
