It is not that the school project is a large threat in and of itself. It is the use of copyrighted material to distribute to the general public and the right of the copyright owner to get paid for the use of their product.
If there is copyright infringement the copyright owner has to deal with all known cases, regardless of "threat" level.
For example, lets say that the copyright owner for "hey hey we're the Monkey's" allowed all youtube and other "incidental" use of the music. Then lets say that someone started putting the song on the internet for free download. If the copyright owner goes after the person releasing their music for free they have one problem.
There are 100's of other times that the copyright infringement was allowed. Though they were only school projects and some people sticking up a video on youtube. Each time an infringement is allowed it sets a precedence for allowing free use and distribution of their copyrighted material.
If the precedence is that you allow free use and distribution, how is this one guy providing the free download of the material any different from the 100's of others that were allowed to distribute the copyrighted material without any action taken?
So, yes, music used in a school project is a threat to music sales because the company has set the precedence for general distribution of copyrighted material without payment.
I hope I explained that well.
The same holds true for patented products. If you patent a product and some guy copies it and puts it in his basement, you have to go after him because you need to set the precedence that copying the patented product in any manner is unacceptable.
If a company starts building an exact copy of your patented product for sale and distribution, but you did not go after the guy in his basement, you set the precedence that it is OK to build copies of your patented ideas.
It is the nature of the legal system.