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!
「中には卒業してから一回も会ってない子もいたんだ」という文章を英語にする場合、次のように表現できます。
There were some I hadn't seen since graduation.
「卒業以来一度も会っていなかった人たちが何人かいた。」
この表現では "some" が「何人か」に相当し、"hadn't seen since graduation" が「卒業以来会っていない」という意味を伝えています。