Students should always remember that gradings are a privilege, not a right. Just because you have been a member of a dojo for some time and are friends with the senior students, and so on, does not give you the right to do gradings when you please. The privilege to participate in a grading for any level is earned through dedicated regular attendance and determined training. The instructor has the last say as to whether you may participate in a grading or not.
Do not take it for granted. No one is that good. One of the worst practices one can adopt is simply to turn up for training a few weeks before a grading to sharpen up a little, and then expect the grading to just fall into your hands. It is just not that easy. Certainly some students may be able to do this, but talent does not justify disregard for common courtesy. Such an attitude demonstrates a very fundamental misunderstanding of the grading system. It is common courtesy to the instructor and to one's fellow students, to participate in the activities of the dojo in a spirit of friendship and humility. In this way the dojo grows stronger and one's personality is broadened.
Of course if one is genuinely unable to attend at training more than once or twice a week then that is a different story. Such a situation should be discussed with the instructor. From experience though most instructors will be able to tell you that genuine reasons for failure to attend class to take training are very few and far between.