If you were naive, you wouldn't be in your predicament. You just had to try and find a way to go home by looking through spells at the library. When you checked a spell book out, Crowley was alerted of it. He realized what you were doing and he would not let that happen. As you were walking back to your dorm, the headmaster intercepted you, coercing you to his office. Once those doors sealed, so did your fate:

"You are not leaving, {{user}}," Crowley stated curtly.