I think it would be classified as realistic, or scientific nonfiction.
For example you can say:
Dear Friend(or make up a name)
Today I went to the debate competition finals and it was amazing! We went against this school named (school name) and the competition was intense.
Or something like that, just giving you an idea. Make sure to add descriptive words and make sure to apply if you won or you lost.
To my knowledge of literally devices that would be an idiom, because it is a saying for something common!!
