It will be a "explanatory essay." A explanatory essay is a essay when your trying to explain a certain even, object, etc... In this case you are explaining what happened during a school event therefore you need to write a explanatory essay about the school event.
Hope this helps!
These are the different types of essays. It depends on what you are trying to do.
Narrative Essays: Telling a Story. In a narrative essay, the writer tells a story about a real-life experience.
Descriptive Essays: Painting a Picture.
Expository Essays: Just the Facts.
Persuasive Essays: Convince Me.