Exams can be a big stress.
The exam season can certainly be a very difficult time for students. Indeed, exams can be very stressful. They can cause you a lot of stress. You can be stressed or even stressed out.
In the context of having all those feelings that make you want to panic, stress is an uncountable noun. You cannot count stress, unless it becomes a particular kind of stress: My father can cause a lot of stress. There is a lot of mental stress on some employees in high-pressure jobs.
Of course, it exists as a verb: He is not a good boss: he stresses (or stresses out) all the staff he is responsible for.
Stressful can also cause some confusion: click here.