The sentence is fine, apart from the apostrophe. An apostrophe is used for contractions or to show belonging. An apostrophe should not be used for showing plurals. Here are some examples to explain.
Contractions: Do not - Don't. I have - I've.
Show belonging: "I went to Clara's house". The house belongs to Clara.
In your sentence, you should just write: "We could go to the movies". Your teacher is right!
- For Teachers