If you are looking for something a little bit different to use with Raspberry Pi's in your classroom, have a look at Kickstarter where I have just launched the 'PiMuxClock'.https://www.kickstarter.com/projects/42 ... pimuxclock
The PiMuxClock, is a simple Raspberry Pi add-on card, to create a basic digital clock and temperature display for your RPi. The key design goal for the PiMuxClock was to get back to basics and develop a low cost, educational hardware project. No buffers or serial interfaces just direct connection to the Raspberry Pi GPIO, programmable with simple Python code to drive the multiplexed display.
There are 9 pledge levels running from £5 (+P&P) for an unpopulated PCB up to an enhanced education pack containing 10 build and tested PIMuxClock boards with lesson plans for £125 (+P&P)