Posts on your page are sorted chronologically. If you wish to keep a post (or multiple posts) at the top of your class page, you can "pin" the post(s).
How to video: pinning a post
Step-by-step: pinning a post
1) When approving or updating posts, a "Post audience" dialog box appears.
2) At the top of the dialog box, a check box is available to pin the post.