Open Question: What should daughter do re: new job?

Dillema: 17 year old daughter has first job at a gymnastics/dance studio for kids. She loves it. Has only worked one day so far. Boss knows she's still in school and will want her to work more when summer comes. That being said, she got a voice mail last night indicating a 'mandatory' meeting this Thursday night from 7-8. Well, because she is still in school, Dance is one of her classes; and she has a big performance that night. They've been rehearsing for over a month. Should she tell her teacher she can't make the performance? She's weary of telling her new boss about the performance because she's so new and doesn't want to appear to be the type that can't be counted on. Missing either seems rotten. What do you think?