I've done indoor group shots successfully with two 300W studio lights in brollies. The brollies spill light all over the shop but you can direct things to your advantage.
I put the key light off to the right but aimed towards the left of the group. This ensures that the weaker edge of the light is lighting up the closest members of the group and the stronger centre part of the light is reaching the furthest members of the group. This creates pretty even light across the group. The second light is used as on-axis fill near the camera position to lift any shadows created by the key.
I can't remember camera settings but I meter across the group and adjust as necessary.
Probably won't do a whole school group but should be ok for a regular class.