educational
This tag indicates that the post is designed (genuinely, or as a parody) to educate or inform the viewer about some subject matter. This is usually a school subject, like grammar, mathematics, science, etc. It is also often in the form of a (faux) textbook excerpt. Many examples (e.g. Danbooru post #5751468) parody the well-known New Horizon English textbook. For such material in a book in the scene itself, use textbook.