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 tree 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 Enciklopedia 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.

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.