"Didn't study enough and anxious about that quiz? Have a Spell literally blow up in your face? Did you get in bed, only to find your biggest Rival sprinkled it with itching powder? If so, you probably have Stress. Everybody gets stressed from time to time. It's when you don't relieve it that it becomes a health problem.
Stress wears the body down. At the end of the day, when you've exceeded your Stress Maximum you will fall ill and loose all your activities for the next day.
You can accumulate an endless amount of Stress, so make sure you take your Rest Action! Resting removes both Stress and Illness. Additionally you can relieve Stress by Actions, Abilities, and Items."
Stress is your enemy from day one.
It plays an important factor in what you can achieve by your calendarial Activities, as incurring an added Stress value exceeding your Stress Maximum will make you stay the next entire day in bed, losing all three activity slots.
It also can bring about Emotions, mostly negative ones.
You can reset Stress fully or partially to your present Stress Minimum value by various activities, most notably the Rest Action that will fully reset it along with your Vitality.
While Stress and Vitality are both chiefly governed by the Fitness Attribute, can both result in losing a full day, and are similarly affected by Resting, the two values are otherwise unconnected.
Neither can you miss Exams as a result of Stress/Vitality, nor can you make enemy Students miss theirs.
Stress and Stress Minimum are two of exactly five values in the game, where "high" means "bad".
The Fitness Attribute x2 determines your original Stress Maximum, which can be it can be positively or negatively Increased or Expanded by other factors, such as skill perks, Clique Abilities, Spells or options at Character Creation (e.g. Prodigy: Physical).
You can learn your current Stress Maximum by hovering your mouse over the stat.
Your Stress Minimum is the lowest your Stress can ever get, even after a full Action:Rest.
Hence: high Stress Minimum = bad
Modifications to Stress Minimum are almost always temporary positive or negative Increases, aside for some Option during Character Creation.
So you will need to manage stress. Stress can be accumulated voluntarily by Actions and Abilities, or otherwise by bungled Adventures, Random Events or Hostile Actions from your co-students, like bullying, usually in amounts from 1 to 3.
You can use Stress as a bonus modifier to all specific Actions and Abilities when planning your daily schedule up to your Maximum Stress Level. This will add the Stress points incurred to the rolls. This does not work where no action can be specified, e.g. during Adventures and Random Events
You will either have to Rest it off, spending one full calendar slot, or use some Actions/Abilities/Spells/Items to decrease it. If there are fringe benefits for using Spells or Actions, this is may be more efficient than just lying down, unless the the amount of Stress is far beyond the scope of those other actions.
The best counter to Stress is having a high Fitness Attribute, as that will give you an esponentially high buffer before you stress out.
If your Fitness is a meek 1, your Stress Maximum will originally be 2, so if Joana comes around bullying you for 3 points of Stress (which is a frequent occurence], you're done. For the latter event it might be wise to plan for a reserve, if your Relationship with her is particularly low, and too for some Random Event.
Stress levels can be manipulated (reduced or increased) by these actions.