Designing Students Exams and Classes Database
My thoughts after a quick review (edited):


An Exam should not have a Day/Start Time/End time. The Exam entity represents the EXAM definition, it should have subject, year etc, NOT the time it is to be sat. I assume from your design that a class can have multiple exams. What you are missing is an ExamEvent entity. This would represent an exam event, where everyone sits down and takes it, NOT the definition of an exam (as the same exam can happen in different schools, by your description). An ExamEvent needs to have an ExamID, ClassCode (think about having a surrogate ID for the CLASSEXAM Entity), Time/Date, LanguageID (assuming everyone sits the exam at the same time in the same language), RoomCode. It should also have its own surrogate key, if needed: ExamEventID. Review the name of this entity.
It looks like you are then missing another key entity - a StudentExam entity, which would have: StudentID, ExamEventID and SeatCode. This represents the student attending the exam and where they are sitting. If each student sits the exam in a different language, but at the same time, the Language entity needs to relate to this entity NOT the ExamEvent entity.
Sticking with what I have specified above, you should then remove the 2nd Student/School link (currently representing the place the student is taking the exam). Instead, using the design I specified, the place the student is taking the exam is defined by the StudentExam object as it links via ExamForClass, through to ClassRoom, through to School.