Teachers and students are permitted to use copy-written resources in limited ways, according to a legal doctrine called Fair Use. For the most part, you shouldn't make copies (including digital copies) unless you are doing the following:  

• reproducing part of copy-written work rather than the whole thing

• using a resource to support curriculum rather than an extracurricular activity

• sharing the resource in a "one-off" way, rather than providing ongoing access

Without the all of above, your Fair Use claim gets dicey. It's a legal grey area. If you're in doubt about whether you're in violation, try this Fair Use Evaluator Tool (by Brewer, Michael and the American Library Association's Office for Information Technology). It can not only help you determine whether you can make a claim to Fair Use but also can provide a time-stamped PDF that you can use to defend such a Fair Use claim.