These days, almost all things are copyrighted the moment they are written, and no copyright notice is required. As long as you limit the use of the copyrighted work inside your classroom walls you are probably operating well within the copyright laws. Copyright is still violated whether you charged money or not, only damages are affected by that. Postings to the net are not granted to the public domain, and don't grant you any permission to do further copying except perhaps the sort of copying the poster might have expected in the ordinary flow of the net.
Fair use is a complex doctrine meant to allow certain valuable social purposes.
Ask yourself why you are republishing what you are posting and why you couldn't have just rewritten it in your own words. Always give proper credit to any borrowed Net Work. If you post a copyrighted work online or take it outside the classroom, contact the original owner of the copyright for permission. When in doubt, the best rule of thumb is to always ask for permission. The student is probably safe if the PowerPoint presentation is for use in your classroom.