Section 110 of the copyright code permits broad uses of copyrighted materials in the physical classroom. An instructor or students can display pretty much any copyrighted work or perform any copyrighted work, live, or via video or the Internet.
Unfortunately this teaching exception does not apply to making copies of any copyrighted material. For that, one would need to rely on fair use. It also does not apply to online instruction.
Fortunately, fair use is available for teaching. An instructor or student would want to look at all four factors of fair use to make a determination if copies can be made for display in a presentation or distribution to students. Fair use looks at 1) the purpose and character of the use (nonprofit education is one of the purposes expressly named); 2) the nature of the copyrighted work (copying factual, scientific, or historical works is more likely to be considered a fair use); 3) the amount (the less that is used, the more likely the use will be considered fair); 4) the effect on the market (does the copying impact profits that could be made on the work by the copyright owner).