1) Classical conditioning can cause test anxiety when a teacher says "tests are coming" (neutral stimulus) and students feel stressed (unconditioned response). Through repeated pairings, just hearing "tests" causes stress (conditioned response). 
2) Using positive reinforcement, a teacher praises a student for on-task behavior (behavior), increasing the likelihood they'll repeat it. 
3) A teacher uses negative reinforcement by allowing a disruptive student to leave time-out early (removal of an unpleasant stimulus) when they calm down (behavior), increasing calm behavior.