school year
Education lasts 7 years. One year lasts 2 semesters. Each of them must be passed to pass the class. Don't worry about the time too much! Some students pass all the semesters at once, some of them take time to do so. It is not a big deal to not to pass a semester or to be stuck at the second year for example. In fact, some of the students are taking YEARS to finish their education. Yet, nobody is bullying them because of this.
how to pass a semester
You need to do the requirements such as passing different subjects. For example - to pass year I semester I you need to pass Basic Potions I, Basic Charms I and Basic Dark Art Defence I. After that, you need to post all your passed classes under a journal: (link)
It will be opening each month near the end of it. It will remain open for one week, then it close for 3 weeks.