Assuming that you are dealing with U.S. high school students I could say something. At Augusta State University I am in charge with AMC 12/10 and Putnam (which is indeed Collegian competition but I alway have one or two very talented high school kids). I put two websites, one for AMC 12/10 ( http://predrag.freeshell.org/AMC/amc_about.html ) and one for Putnam ( http://predrag.freeshell.org/Putnam/putnam_news.html ) 

Carefully open all links on the bottom page with resources. My favorite however (probably I am nostalgic a bit) is link to Kvant ( http://kvant.mirror1.mccme.ru/ ). Serge Tabachnikov of Penn State have published two books of translations of most memorable articles. 

I also like to use Gelfand's high school text books 

For example Algebra 
http://www.amazon.com/Algebra-Israel-M-Gelfand/dp/0817636773


I like very much articles from three volume translation of 
Mathematics: Its Content, Methods and Meaning by A. D. Aleksandrov, A. N. Kolmogorov and M. A. Lavrent'ev. 

http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=mathematics+its+meaning+contectst&x=0&y=0


However, the articles are very, very challenging.

There is also a very famous Encyclopedia of Mathematics for high school students in Russia but I am not sure if it is translated into English.

Look for Vinogradov's book on number theory. It very, very deep but accessible for high school kids.