The Open Academic Robot Kit

The AX-12A servos will take up to 12V so powering them from a 3 cell LiPo or LiFePO4 battery is quite doable. In a classroom environment, the LiFePO4 batteries are probably a better option from a safety perspective. The slightly lower voltage shouldn't be a major issue.

It is highly recommended that a battery warning circuit be installed - ready-made versions for model aircraft cost a few dollars and will prevent you from damaging the battery by over-discharging them. Make sure that the one you get works on 3 or more cells. Some of them only work on 2 cells because they're designed for model aircraft where the battery only needs to run the radio (and propulsion comes from an internal combustion engine).