And yet, that's pretty close to the method that all Primary and Secondary schools use in the developed world. The student shows up to class, the teacher instructs, then the student takes a test, usually with little to no input from the teacher as to whether or not the student is "ready."
Ask yourself what is the point, the purpose behind the test? Obviously to ensure that the student has a working knowledge of some given skill and information set and can perform at some given level of expertise. Sometimes, it's important to the participants that the student be tested "under stress." Now, look around at all the various training programs testing physical skills and information from around the world and across disciplines. Look at everything from ASE (automobile mechanics) to Medical certifications (Doctor, Nurse, etc.) to computer technology (CompTIA, MCS[P/A/E], RHC[A/E], etc.) to legal (Lawyer, Paralegal, etc.) to college & university. What you see is a variety of training and testing methods. In some cases the student has no choice when to take tests and must meet a deadline to test, pass or fail. Failure means taking the test again, maybe repeating the curriculum. In some cases the student may take all, some, or only the final certification test at their discretion, with any input from the instructor being at the student's discretion as well to heed or not. And there are many variation between. All of them are proven to work, to produce capable graduates with serviceable skills.
If all of these various methods can work for other skills instruction, then it can work for martial arts too. It is entirely possible to run a martial arts school like a regular primary school where the students have a curriculum that they must complete and then, with no discretion at all from the instructor or the student, the student takes a test at a given time. Success or failure could determine whether or not the student must repeat the curriculum.
It is entirely possible to run a martial arts school like a Microsoft Certified Professional program where the instructor teaches the required information and the student decides when he wants to take a test. The test may be performed on site or the student may even have to go to a third party to take the test.
It is entirely possible to teach martial arts in the old European "Apprenticeship" model with the student passing from Apprentice, to Journeyman, to Master. In fact, martial arts instructors in England used almost exactly that method during the Tudor period with the
Company of Maisters. Students would petition to join, train for no less than 7 years, then would be required to request permission to "Play the Prize" which, if granted, included posting an open challenge to any and all other members in the Company. They would be judged by a panel of instructors based on their performance. 7 years minimum from Scholar to Free Scholar. 7 or more from Free Scholar to Provost. Provost to Master, if ever, another 7+ years. And the student would decide if he thought he was ready to Play the Prize but had to ask permission from the Master, who could give or withhold the permission. (Students would also be required to buy, out of pocket, the challenge hand-bills and post them around the town and in nearby locals).
Of course, rank-by-immediate-award (Batsugun) still happens occasionally in some arts such as Judo and is (or was) quite common in some BJJ schools; when you can "hang with" the next rank, then you achieve the next rank.
Stop thinking so narrowly. Just because some other school does grading differently from how you are familiar with it, doesn't mean it's wrong. It's how they do it. Results are what matters. If, in the end, the student is transmitted and retains the requisite knowledge then their method works, regardless of how much input the student has on when he attempts to grade.
Peace favor your sword,
Kirk