My first tip is to make sure that your Sim has food on them. This really only applies to those who do a heap of classes at once but honestly a little something from a vending machine can make a difference in their moods. Sometimes it takes too much time to get home and make something to eat by the time your next class is calling for you and so a simple candy bar will fill you until dinner time and leave some more time for you at home to do a bit of study until you really need to eat.
I constantly had my student Sims complaining at first when they were during three lessons back to back because they'd get so hungry but by the time I've got them home to eat it's time for the next class! Now I get them to keep some food on them so they can just eat and dash.