The Immigration Office will not extend your visa if you show a poor attendance record and/or low grades. You are allowed to miss up to 5 days of class, this is 10 hours if you are a regular student. On top of that, you are required at least 5 additional hours per week from the supplementary classes as well. Lacking hours in any category will be detrimental to the status of your visa renewal.
Grades: While there is no set requirement on what score percentage you must get, it is advised to just do your best and score decent enough. When renewing your visa the Immigration Office will see your progress and if your grades are poor they don't see a valid reasoning in extending your visa for longer stay to study. Make sense? In other words, attend class and don't slack off.
Note: If you plan to study for more than 2 terms you can qualify to apply for a Resident Visa (ARC) at the Ministry of Foreign Affairs after studying for at least 4 months. Again, you will need to bring the items requested to them and attendance and grades is looked heavily upon. Please refer to the Visa section for more information.
So according to this example, I have missed 2 days of class (4 classroom hours) and 1 hour of supplementary hours for the month of december.
Intensive class student:
You shouldn't be missing anything for then 3-5 classes out of the quarter since each days worth of class consists of 3 hours. They are intensive for a reason and you are given daily Listening/Writing quizzes as well as short essays, projects and "4-6" hours worth of homework/study time per day. If you are going to be missing anything more than that, then do not consider taking an intensive course unless you are serious about it.