Final answer:
The phrase 'let the cat out of the bag' is an idiom used to describe someone revealing a secret, such as a student telling her parents about her alternative college plans.
Step-by-step explanation:
The phrase 'let the cat out of the bag' is an example of a commonly used idiom in the English language. An idiom is a figure of speech where the words together have a meaning that is different from the dictionary definitions of the individual words. In the context provided, the idiom means that someone revealed a secret, which in this case, refers to a student disclosing her alternative college plans to her parents.