Why can't I say I?

Through out literature and language arts classes in middle and high school I've noticed that most teachers say the same thing. "During this course we want to teach you how to write with your own voice and style." However about 3 days in to said course we are given a list of a bunch of things we aren't allowed to write: first person, second person, absolutes, clichés, "childish" examples, rhetorical questions, and even adverbs. It's a little hypocritical in my opinion. They want us to write in our own voice, except for these certain parts of our voice. They say that it's an immature way to write but what if I'm an immature person? Maybe i like clichés. Maybe if i was talking i would use a cliché. Or an adverb. I would definitely use a childish example. Maybe that's my voice. I think this system of limiting what a person may or may not say in a paper is resulting in an overflow of papers that while differing in subject, are in essence the same style. It's boring. It gets very old VERY fast. Maybe instead of trying to not sound immature, you should try taking a step on the "wild" side. It might brighten up these barren wastelands you call papers.