Although copyright law has many exceptions for educational purposes, there's a big difference between what you can do in the classroom and what you can do outside of the classroom. When you are in the classroom, you are free to perform or display any copyrighted works that you have legally obtained. Playing videos and music, displaying images in your presentations, reading texts aloud—all of these are examples of things you can do in the classroom without any restriction. However, outside of the classroom, you are more limited in what you can do. This is especially important to keep in mind if you plan to copy and republish copyrighted works on the open web.
Not everything you find on the Internet can be copied and reused without restriction. Even if you do not see a copyright notice, it is likely that the work is copyrighted. However, there are many resources you can use to narrow your searching to content in the public domain or content that is available under a Creative Commons license.
If a work appears online with a statement that it is licensed under Creative Commons or in the public domain, you should still consider whether or not these claims are trustworthy. Doing a small amount of research on the work's origins or avoiding using works that are clearly copyrighted (for example, works from recognizable publishers or corporations) in nonpermissable ways will help you avoid infringing copyright.