This is something that is part of the Williams application. My take is they are more interested in quality of writing than thought or content. I wouldn’t overthink it. This is what they say:
“Williams does not require a writing supplement. However, some students feel they do their best writing in an academic context rather than through narrative pieces like admission applications. If you are interested in submitting an example of your academic writing, you may share a 3-5 page paper written in the last year. The paper may be creative or analytical, can cover any topic, and need not be graded. Please include a description of the assignment or prompt. Please do not submit lab reports.”
Our school does mostly in-class writing in 11th grade as they are preparing for APs. These tend to be shorter. My son submitted an essay that was about 2 pages typed. With the prompt description, he included a note that it was an in class piece, per the curriculum.