My girlfriend wants to do something for Valentine's Day but I plan on staying home to do my homework instead.
I think you need to make a big decision. What's more important, your girlfriend or homework?
Doing homework is more important to me
I'm sure your girlfriend will understand that getting the homework done is more important than going to the dance. Explain the situation to her, and try to plan a special dinner date for another night. She will likely be more than willing to compromise with you, so that you can get your homework done.
What about being responsible for your actions and commitments.
I get where you're coming from, we need to stay firm to our commitments. But if we are not able to make good on those commitments it's good to make it known.