The entire percussion section with the high school band. march; marches; play; or playing

The correct answer should be either marches or playing. That's because the marching band is considered to be It and it goes together with -es because it's third person singular. If you use playing then you build a subordinate clause that needs something else to fill it but it is correct either way.

Identify the theme of the story in your sentence

Pardon me. to answer questions like this, we would have needed more information. for starters, two sentences from the excerpt that came with the question.