A cheap and simple way is use 2 battery packs, one for 7-12, another for 6.5. It not elegant but its fast and cheap.
You make your own battery pack out of 1.2V NiMH x 10, the top of the pack is 12V and tap at the 5 -6 cells for lower voltage.
If you need stricter voltage regulation you'll need a voltage regulator for each, as needed.
if you need compact elegance, then a single battery pack supply with a regulator feeding each of your demands, but if you don't need that yet, why go there and spend more, unless its part of the class exercise.