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！
Using the Korean language I’ve been studying since high school.
By applying the Korean skills I’ve developed since I was in high school.
「高校生の頃から勉強してきた韓国語を使って」と表現したい時は、 Using the Korean language I’ve been studying since high school. （高校生の頃からずっと勉強し続けてきた韓国語を使って。） と言うのが自然です。
since high school を使うことで、長い年月をかけて継続してきたニュアンスがよく伝わります。
カナダ 
アメリカ合衆国 
