Typically, a student has three main reasons for stress: the educational process itself, relationships with the class and relationships with teachers. First, I will support the previous commentator: you need to prepare for classes so as not to get nervous at the desk or at the blackboard. Secondly (and this is the main thing), you need to outline for yourself the goal of the entire educational process, to concentrate on what you need to enter the university. If you have not chosen a specialty, choose and make every effort to pass the required exams. The relationship with the class and the teaching staff is something that can (and, if the situation requires it, must) be ignored. The main thing is to calm down, accumulate strength, collect knowledge in the chosen disciplines and calmly, measuredly move towards admission.
Just simply get ready for classes. School material is not so complicated that even the stupidest person does not understand it at the minimum level. And if a person is prepared for classes, then there will be no stress. Based on my experience, I say that if you are ready for the test / exam / oge, then there will be no stress, and having received high scores for these types of work, you will get a sense of a certain pride and joy, rather than relief after a stressful situation