Usurpation means taking someone's power or property by force. Locking the teacher outside of the classroom and taking charge of math class is a form of usurpation.
It may help to remember the word use when thinking of usurpation, which is what happens when you take someone's power or belongings and use them for yourself. When Shakespeare's Macbeth kills King Duncan and replaces him by usurping the throne, that is an act of usurpation. Stealing a person's personal information in order to open up a credit account in his name also counts as usurpation.