As sezme suggested the word imposing is not necessarily incorrect but it just doesn't sound natural. Also, you do not have to precise it was something you imposed your students since the context helps to understand this, this is the beauty of implicit meaning 😊. In other words, you do not necessarily have to explicitly say that the essay is mandatory since you are giving it to your students in class. In that context, when you give an essay to your students during class, you can guess it is a mandatory task. I hope it is not too confusing, you can send me a message if you need further explanations 😊.