You can easily “bookmark” the posts you want to find easily. To do so, you just have to click on the arrow at the upper right of the post and select “bookmark”. The bookmarked posts are personal. You are the only one to see the bookmarked posts.
Only the teachers can pin a post, it is a selection of posts the teacher would like to put forward. This explains why all the selected pinned posts are visible in the “pinned” section.