There were some people there I hadn't seen since we graduated from high school.
There were a number of people there I hadn't seen since we finished high school.
ーThere were some people there I hadn't seen since we graduated from high school.
「高校を卒業して以来会っていない人が何人かいた」
There were some people there I haven't seen since ... で「そこには…以来会っていない人が何人かいた」
ーThere were a number of people there I hadn't seen since we finished high school.
「そこには高校を終えて以来会っていない人が何人かいた」
There were a number of people there I hadn't seen since ... で「そこには…以来会っていない人が何人かいた」とも言えます。
ご参考まで!