Every May you have to take exams to pass into the next year. You have to take an exam for every course you signed up to, if you don't take those, you fail that course.
The examinations are always held at the Great hall, where there is a subforum for NEWTs. OWLs and Years 1-4. In each forum, there will be a topic for each class. You will find your exams in those topics. Once you complete the exam, PM it to the teacher, DON'T POST IT. You can RP in those topics but not post your exam there.
Your grades are then sent to you once all examinations are over.
Heres how the grading scheme works:
For years 1-4 you have to pass at least 4 exams to go to the next year. Passing means getting at least an Acceptable. Anything less is a fail grade. If you pass 4 or more exams, you go into the next year and continue all the courses you took.
For OWL or NEWT students, you have to get at least an Acceptable to pass or graduate. If you don't, you fail and drop that course to the next year. However, as long as you pass 1 course, you have the option of going into the next grade.
The exams end by the end of May, so if you don't do your exams by then and PM your teacher, you fail and don't go into the next grade.