Students! Or, more accurately, Student Workers! The are a blessing in that we couldn't get our jobs done without them. They are a curse in that the are Students FIRST!
You can't manage them the way that you would an ordinary worker. When a student worker is scheduled for 2 hours per day on Tuesday and Thursday, and doesn't show up you have to assume that there is a study session or a test the next day. You can't just say, "You weren't here. You're fired." That tactic would leave you struggling on alone with no help at all.
When the absent worker appears on FRIDAY morning - you have to look at it as if they are keeping their part of the bargain. They promised that they would give you 4 hours and you are getting 4 hours - it just isn't on YOUR time table.
This morning I was expecting to train two new workers. I had set aside time in my day to spend in the lab with them. I would be teaching them something new. Something that they will need to be able to do alone in a few weeks.
I get to the lab this morning. Tuesday/Thursday's missing worker is on task - doing the job that had been set aside for the expected Friday workers. She already knows how to do this task and she is doing it correctly and efficiently. The other two do not know how to do this task. By the time they get here it will be done. When they need to do this in the future, I may not have the time in the lab that I can dedicate to them exclusively.
As soon as I finish writing this I will spend the next little time at my desk trying to figure out something for the two new workers to do that will teach them something that will make them more productive, independant workers in the future...