Create a family schedule for the week that incorporates all required activities for each day. Examine the time left each evening beginning from the time the girls get home. Identify blocks of time that are available, rather than just one long study block of time. By breaking the time up, the tasks can be approached with more energy at the beginning of each session.
Using an agenda or planner, have your daughters make a list of work that has to be done. Let the girls do those sections that are easiest for them at times when you are not available to help them. Check off each task that is done for a sense of accomplishment.
Have the girls note a starting time and an ending time for each of the tasks. By knowing realistically how long certain tasks take, they can allot appropriate time for future assignments.
Take a break after each task. Even a brief time to stand up and stretch before moving on to the next assignment feels good.
Keep all interruptions to a minimum. By making the schedule up in advance for the week, your daughters can tell their friends when not to call, and you can try to allow a block of time devoted to family fun each night, in addition to study time.
Usually the dread of doing the work is far greater than the work itself. When your daughters begin to think of their evening being taken over by work, I am sure they blame school for disrupting their family time and their social free time.
You have shown them real support by contacting the school and trying to gain information about what is being assigned. By keeping the time-task log for each girl, you can re-approach the teacher with this information. The teacher can suggest additional strategies that can reduce time spent on work without reducing the quality of the job.