Should I take on this one-time project?

I'm a makeup instructor who's just starting to do more freelance makeup worker, and a colleague of mine asked me to do a freelance project with her. It would be great experience and great for my portfolio. But I'm scheduled to teach that day! And another teacher just informed me that she'd like to take off that day too! aaarrrgh... I rarely ask for the day off. Should I ask for the day off and take on the project? Or should I be the steady eddy and work as scheduled? I'm conflicted.