Bench tests are graded on the work in front of the examiner, not on effort or intention. Knowing exactly what a grader looks at is the difference between practising hard and practising usefully.
Graders assess the preparation, not the process
Nobody watches every move you make. What gets assessed is the finished preparation on the typodont. That means a preparation which was difficult but ended up correct scores well, and one which felt smooth but ended up over-reduced does not.
Margins
Margins need to be smooth, continuous and well defined. Most schools expect the margin to sit 0.5mm to 1mm above the gingival contour.
The common failure is not a rough margin. It is repeated polishing at the end to try to smooth one. Each pass removes more tooth, the margin becomes jagged rather than smoother, and it drifts down towards the gingiva. Damage the gingiva and you lose marks twice over.
Occlusal reduction
Reduction is checked against the guideline for that preparation. Too little and the restoration would not have room. Too much and you have removed tooth that did not need to go.
Worth knowing: graders do not rely only on your putty index. They can also compare your preparation against the adjacent tooth and judge depth by eye. Verifying your depth more than one way as you work is the habit that prevents this.
Taper and wall form
Walls are assessed for consistent taper and adequate retention form. Both extremes cost marks. Over-tapered walls lose retention. Walls left too close to parallel, which happens most often on bridge preparations when a candidate is chasing a perfect look, create their own problems.
Taper is really a question of handpiece angulation and having a fixed reference point. Get those right and the taper becomes repeatable rather than a matter of luck.
Damage to adjacent teeth
Nicking an adjacent tooth costs marks. It is recoverable.
Trying to polish the damage away so nobody notices is not. It almost always makes the damage worse, and it turns one deduction into a bigger problem. If it happens, tell the faculty. Identifying your own mistake is treated very differently from concealing one.
Temporary crowns
Where a temporary is part of the exercise, it is assessed on fit, margin, contour and finish. It is also where timing shows. Candidates who have never fabricated one under exam conditions almost always take longer than they planned.
Time management
Time is part of the assessment whether or not it appears on the mark sheet, because an unfinished preparation cannot score well. Most candidates lose time in the middle of the exercise and then rush the finish, which is exactly where the visible marks are.
What this means for your practice
Practise the specific preparations your schools set, under time, and get someone experienced to assess the result the way a grader would. Self-assessment has a limit: you cannot see your own work the way an examiner does, and repeated unassessed practice can make a fault permanent.
The full list of what most often goes wrong is in why candidates fail the bench test. For evaluated practice, the course with Dr. Kaur includes assessment on the work you actually produce.