Most of the items collected by the Smithsonian Research Online program are written by and for scholars. These publications can be quite technical, dense and specific to a particular field and its experts. However there is one collection of digital material that is both scholarly in nature but also easily read and understood by non-experts. The monthly, Letter from the Desk of David Challinor was produced between 1989 and 2008 and includes over 200 well-written, interesting pieces on a variety of topics such as forestry, evolution, animal behavior, anthropology and the history of natural science. What's best is that these 2-3 page essays, although authoritative, are written at a level which can be understood by most high school students.
Dr. David Challinor (pictured here) was a former assistant secretary of the Smithsonian but spent his later years at the National Zoological Park where among other things, he wrote his monthly Letter. It was distributed in print to a variety of people, mostly friends and associates of the Institution. Later in his life he distributed the Letter electronically via email to the same group. It is interesting to note that he continued to write until just before he passed away in March 2008.
All of these essays are collected in the Smithsonian Digital Repository, a component of Smithsonian Research Online, managed by the SI Libraries. You can search the full text of Dr. Challinor's letters here.
Give it a try. You might be surprised at what you learn.