I was able to use the Korean I had studied since high school to take care of a Korean customer the other day. It was such a nice reward.
この場合、次のような言い方ができますよ。
ーI was able to use the Korean I had studied since high school to take care of a Korean customer the other day. It was such a nice reward.
「先日、韓国人のお客様の応対をするのに、高校の時から勉強してきた韓国語を使うことができた。大変やりがいを感じた」
to use the Korean I had studied since high school で「高校の時から勉強してきた韓国語を使う」と言えます。
ご参考まで！
カナダ 
