There are lots of great PowerPoint resources on this site. Select the link to "Countdown Timers for PowerPoint" to get a wonderful variety of free downloadable PPT timers that you can customize to meet your needs.
This PDF, created by the Loudoun County Assistive Technology Team, shows how to create a basic countdown timer in PowerPoint. Countdown timers can support time management and organization. Add your favorite graphics, such as a school mascot, to customize it for your students.
This free downloadable PowerPoint slide show created by Dave Foord includes a wealth of PowerPoint countdown timers that you can customize to meet your needs. Great for time concepts, time management, and organization!