Clare hadn't just picked this dress by chance. It had to pass a test in order to be chosen! Her school had rules about what was acceptable in the form of dresses on the night. Specifically, if Clare's dress was too short, she wouldn't be allowed in.
The rule was that the dress must be longer than your fingertips when your arms are by your side. Clare had tried the dress on, stood up, and let her arms hang. The dress was longer. No problem.