OK, then my first idea clearly won't work, though my second one might, depending on what you have hidden in those storage drawers and how skilled/motivated the student is.
Make a simple 2 digit 0.0-9.9V digital voltmeter, would even be useful later if done right. You'd need two 7 segment LED displays, or 30 individual LEDs, plus a pair of 74HC595 (or similar) 8 bit shift registers.
Simple voltage divider on input, 200Kohm or so in total. May have to tweak the lower resistor in parallel with the A/D input to account for load of A/D, perhaps talk to students about calibration and accuracy here?
Pair of diodes at the A/D input pin for over-voltage/reverse polarity protection, small cap in same location for a bit of noise immunity.
Output shift registers drives the LEDs directly through current limiting resistors. Either use 7 segments, or 2 LEDs in series for each segment plus decimal point. A 74HC595 can sink or source up to 70mA through Vcc or GND, so you can use either CA or CC displays, depending on what is available. I'd start with 8 mA per segment/chain, test. 2 LEDs in series on a 5V supply may be tight, but should be doable, just, with a CMOS driver plus small current limiter.
The input can be made more fancy to allow it to detect reverse polarity, which the display could show in a suitable fashion, '--'. Would give opportunity to talk about the potential problems with common ground etc.
On a 10 bit A/D you could even 'autoscale' the 0-1V range for 10mV resolution as a bonus software feature, movable decimal point and all, if the student gets the basic functionality up and running too quickly. Questionable accuracy perhaps, but fun extra.
Pin budget: 3 for control of the shift registers, 1 for voltage input, 1 optional for reverse polarity detection. Still leaves RESET for debugging and reprogramming. Could also use an AVR with a higher pin count for this project, if they are available. Would save the shift registers, so may be a more economical solution.
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