When I left for college, I did so with the conviction that my "real" friends were my high school friends.

【答】这个句子很简单。

1. I did so 实际是重复前面句子 I left for college 的意思。

2. with the conviction 意为“带着这个信念”， that my "real" friends were my high school friends是the conviction 的同位语从句。

3. 由于I did so 跟 I left for college 意思重复，所以，不必再翻译一次，而把“带着这个信念”直接翻译为“坚信”，把that my "real" friends were my high school friends 按宾语从句翻译。

【翻译】在我离开家乡上大学的时候，我确信我“真正的”朋友是我高中的朋友。