Copyright will be infringed where you use material (ie you exercise one of the exclusive rights of the copyright owner) in way that is not permitted by a statutory or voluntary licence, one of the copyright exceptions or without the permission of the owner. Most teaching activities will be covered by a licence with a collecting society or a copyright exception.
A copy does not need to be exact or identical to be an infringement. For example making a three dimensional copy of a two dimensional artistic work or vice versa will be a copyright infringement. Copying a significant extract of a copyright work will also be an infringement.
Copyright may also be infringed by a person who authorises someone else to do an act covered by copyright without permission. This means that the copyright owner may sue both the person who authorises, as well as the person who did, the infringing act. This means that school can be liable for acts done by their students, if they authorised the activity.