Should I list all of my scientific writings and technical reports (including those written for coursework) on my resume? I am an undergrad preparing my resume for grad school application. During my undergrad I have done many projects, including a few real research projects and also those for courses as final projects, that were just about designing something rather routine (a simple robot for example), or a compilation of other research works. Most of these class projects have had a report (a writing).

Since I have just one official peer reviewed conference paper, is it a good idea to list all those technical reports in my resume in the Publications section? Or, conversely, adding these may decrease the value of that peer reviewed paper and making the resume look unprofessional?
As Sverre says in a comment, you should make sure that your peer-reviewed publication stands out. It is common, in an academic CV, to put such publications into their own separate section.

As far as your other technical writing, my inclination would be to include these only if they are available to the public in some way, as opposed to just being pieces of coursework. For example, if you have an undergraduate thesis which is lodged in your university library, or have (co-)written a technical report which is available from your department, then these should be included in the CV. They are a class of publication. But essays and reports that were only submitted for assessment are not the same sort of thing. You can certainly mention them in the part of your CV where you talk about your various courses and projects, but it would be misleading to list them among your publications.