Secondary school students can benefit greatly from many of the Open Source packages that exist. Below we list a number of ideas that will provide interest to many students in this age group.
Develop a WIKI for the students to play with. See what they can create cooperatively. See these for more information about WIKIs.
Develop a news site/portal for the students to contribute to. Students could use it to serve weekly news and things of interest.
Encourage the students to develop webpages of their own, these could be personal or project related.
Let the students create 3d artwork with blender. See here for some examples of what's possible:
Encourage students to improve their typing ability with Typing Trainer.
Allow students to prepare for chemistry lessons and to practice in their own time by teaching them how to use GenChemLab an application intended to simulate several common general chemistry exercises.
Help students learn how to solve linear equations and systems with Ginette.
Teach students artifical intelligent concepts by demonstrating CTF a multi-agent capture-the-flag framework.