I think if I had had more friends in high school, it would have been more fun.
High school would have been more enjoyable if I had made a few more friends.
ーI think if I had had more friends in high school, it would have been more fun.
「高校の時もっと友達がいれば、もっと楽しかっただろうな。」
ここでは過去の事実と反することを言っているので、仮定法過去完了形を使って表現できます。
If I had 過去分詞 ..., it would have 過去分詞 ...
ーHigh school would have been more enjoyable if I had made a few more friends.
「もっと友達を作っていれば、高校はもっと楽しかっただろう。」
ここもif節が後ろに来ていますが、最初の例文と同じく仮定法過去完了形を使っています。
ご参考まで！
I think I would've enjoyed high school more if I had more friends.
High school would have been more fun if I had more friends.
こんにちは。
下記のように表現することもできます。
・I think I would've enjoyed high school more if I had more friends.
もっと友達がいたら、私は高校をもっと楽しめただろうなと思います。
・High school would have been more fun if I had more friends.
もっと友達がいたら高校はもっと楽しかったと思います。
enjoy は「楽しむ」という意味です。
ぜひ参考にしてください。
カナダ 
アメリカ合衆国 
