Many administrators don't realize that music technology exists, much less that it will help their music teachers teach music more effectively. You will have used what you had to better your music program. Now, maybe the powers will take another look at your situation, and perhaps make more technology available to you to further maximize your teaching. When considering a projection unit, you are faced with several choices. This would involve purchase of a video card or other hookup for the back of your computer.
Many laptop computers have composite TV output capability built into it.
Another point to keep in mind. Always search for ways to actively involve your class with the one computer. A computer show run by you or one student is not going to get the job done. You need to come up with ways to involve all of the students. Competition is usually a great way to make the participation happen.