For example, there are essential tasks and jobs too. From factories and power plants till hospitals.
During WW1 and WW2 for example medicine students ( nurses, doctors etc related ) had to work in hospitals in their free from studies time. Yes. A lot even volunteered to do this for free, some % from them even had donated stuff they had or money.
The same technical stuff , chemistry etc : a lot of students ( unis, trade schools ) had continued and were used in factories, labs, power plants etc... stuff called shifts. 12 h, 24 shifts etc. As long as it take ....
If government can't implement real wartime economy it can't win large scale war.