We hire a number of temporary employees on a contract for 130 hours per 4 month term. Due to the organizational structure, employees are paid for 130 hours whether they work them or not. A vast majority of the required hours are to be spent in a lab, in person. As a result, we can't ask one person to attend the same lab twice. We have a large number of employees, and once we've allocated all of the hours we see that almost all of them are working only 100 hours while one or two per term are required to work around 116 hours. This frequently leads to conflict and a feeling that someone is not being treated fairly.
The long term solution is to re-organize so that we hire more employees on 100 hour contracts. However, I can't fix that this term, and employees are complaining.
My answer to the employees is that they were contracted to work for 130 hours, and it is not required that I allocate the hours equally. This bothers the employees, since they feel that they are being required to work more for the same pay, and it bothers me since the only reason these particular employees were required to work the extra hours is because they happened to be available.
What are some strategies to handle this, and smooth relations with the employees?