Barty Crouch Jr, impersonating Mad-Eye Moody during Goblet of Fire, demonstrated the three Unforgivable curses to the fourth years during Defence Against the Dark Arts. His reasoning behind this:
‘How are you supposed to defend yourself against something you’ve never seen? A wizard who’s about to put an illegal curse on you isn’t going to tell you what he’s about to do. He’s not going to do it nice and polite to your face. You need to be prepared. You need to be alert and watchful.’
Goblet of Fire - page 187 - Bloomsbury - chapter 14, The Unforgivable Curses
Crouch Jr/Moody also says during Defence class:
‘Now ... those three curses – Avada Kedavra, Imperius and Cruciatus – are known as the Unforgivable Curses. The use of any one of them on a fellow human being is enough to earn a life sentence in Azkaban.’
Goblet of Fire - page 192 - Bloomsbury - chapter 14, The Unforgivable Curses
In class, Moody systematically places each student under the Imperius Curse:
Harry moved forward into the middle of the classroom, into the space that Moody had cleared of desks.
Moody raised his wand, pointed it at Harry, and said, ‘Imperio.’
Goblet of Fire - page 204 - Bloomsbury - chapter 15, Durmstrang and Beauxbatons
Goblet of Fire indicates that Crouch Jr/Moody's lessons were interesting, exciting, and unique -- would word get back to the parents of any of the students that Crouch Jr/Moody was performing the Imperius Curse on their children? Why did Dumbledore allow this?
Why wasn't Crouch Jr/Moody arrested and placed into Azkaban for casting the Imperius Curse on students?