You have your own room?
Then I'd loft the bed. Get another chair. Throw down an oriental rug on the floor. Set up a small dry bar. Get a mounted animal head or antlers. A few etchings. Then maybe a projection TV.
Posters. Posters of what ever you like. Some may disagree but honestly, its the last time in your life you'll be able to hang posters in your room without someone giving you grief about it:)
Frame them, and they'll look more like a "man cave" than a teenager's bedroom. Actually, if they're nicely framed, you won't get grief about them later on, either (assuming the content isn't just embarrassing).
Yes, framing makes all the difference.
Use some manly decor. Books, shaving supplies, hunting/fishing stuff, paintings, mechanical things, stuff like that. Things that can be useful but still look nice on shelves.
For school, go minimal, a good work of art or two. The rest should be functional.
You may want to invest in some good lighting that you can adjust to your needs.
I would suggest bluetooth headphones that are very comfortable. This will let you cut out the twits who play to loud and study with.
I would also suggest a poster you make yourself. Put on it your values, your Goal, IE your Major with GPA and one personal slogan to help you focus on achieving that goal.
Personally I had a timeline of courses to achieving that goal. But I also had way to too much clutter that helped brake my focus. It is easy to fill up the room as a "man cave" but you are not there to relax and unwind, so much as grow and get a marketable degree to give you access to a career.
A library of the best porn available with a big mofo TV and massive amps to blast the grunting and groaning throughout the building.
A 100 megaton nuclear stereo system with the best of Motorhead playing around the clock and the amp turned up to 11.
A man trap in front of the door for all the fucks who complain.
And an iguana.
The one thing that made my room feel like a 'man cave' in stead of a 'room' was the addition of another arm chair. This way I can receive another person in my room, and sit down with them. Somehow it feels less 'manly' to sit on a couch with another dude, and more manly to each have your own chair. It also facilitates me getting away from my desk when I have a guest, eliminating the temptation to 'quickly put on this song, or this filmclip or whatever'.
Create a place to talk and have a drink. A man-cave is useless if it can't be shared.