When I was working with professors as an undergraduate, I wrote up progress reports and presented my work to the research group and collaborators about once a month. It would not have been appropriate for me to list these as publications.
What makes a publication different from an informal report is that the publication has passed a peer review; another mathematician has looked at your work and vouches that you have an interesting result.
If your reports contain results that would be considered publishable in academia *and* the information is not classified, I would certainly mention it somewhere in the application. Depending on how formal your papers were and how many people they were shared with, your personal statement may be a more appropriate place than the publications section. If you are in doubt, seek the advice of your professors; they are usually happy to help.