The Jefferson Lab has some useful apps for the high school chemistry student to hone their elements/balancing skills.
I give pictorial instructions on how to access the site (and a link to click on the bottom, if you give them electronic access to the pdf...).
This worksheet gives students a guided format to explore the "It's Elemental" on-line chemical equation balancing activity. It is designed to be self-explanatory for maximal student inquiry and minimum teacher explanations.
I include a metacognition piece at the end (always a good thing in my opinion...)