Is my usage of the colon in this sentence accurate?

In the sentence in the description, should the semi-colon be a comma, or remain a semi-colon?

• In this sentence, "The teacher was nice; my classmates weren’t bad, but once I saw the reading list I knew I wouldn’t like it." is the semi-colon correct? I originally put a comma, but my cousin corrected it to a semi-colon when she read it. I always thought a semi-colon was put at the end of a sentence, but not to end it and so that you can put a related idea on the other side of it (if that description makes any sense to you...) I asked my sister and she said and would be a better choice, but to definitely not put a comma. If a comma doesn't fit here, can someone explain to me why please? Thank you! It's from a paper I had to write for a comp assignment.

• Answer:

  Semicolon is fine. A comma wouldn't be right because the following phrase is a different idea. You could also use a period here - that's one way of deciding you can use a semi-colon, when a period could also be used.