Wayne, first let me apologize for the delay in responding to your post. Flight schedules interfered with my computer access, and I just now logged on to the discussion board.
I commend you for seeking help with this behavior issue and understand your concern that this issue has become high profile with your evaluator. Because of that, I recommend that you keep meticulous records about the behavior in each of your classes so that it is clear to your evaluator that your classroom management is working in 4/5 of your classrooms and with the vast majority of your students. When you are able to say (and back up with written records) that X % of your students have had no discipline issues, that is a clear indication that classroom management as a whole is not an issue for you. Also keeping clear records of what you have done to address the problems in the one challenging class (including seeking administrative assistance and how you attempted to implement the suggestion that you remain consistent with negative consequences and the results of that implementation), shows that you are willing and able to adjust your classroom management to meet the needs of your students. It will also show that consistency with "punishments" has not worked.
Let me clarify that I have no experience with high school agricultural mechanics, so I hope some teachers with shop and other hands-on types of classrooms will chime in.
Since you have narrowed down the problem to the leadership of three seniors who refuse to do the work required for the class, I wonder if these boys need the class to graduate. If they don't need the credit, they have no motivation to work and lots of motivation to indulge their "seniorititis" and goof around. Did they just start the class at the beginning of a new semester, or have they been with you for awhile? Do they have similar issues in their other classes? If not, perhaps they are just filling up space to get a credit and graduate. How can they get the credit if they are not doing the work? If they don't need or want the credit, why are they in this class which appears to me to be an elective?
As far as the other students, it might be productive to have a conversation with the class (or the individual students who have allowed themselves to be negatively influenced by the three negative leaders) about how their behavior is affecting their progress. Let them know that they are missing out on opportunities for doing shop work. Let them know exactly what needs to happen before they can do that work. My guess is that most students would prefer the hands-on experiences and when behavior is connected to that kind of positive consequence rather than the negative punishments, you are likely to get better results. Is it possible to make a plan in which doing specific things leads to something they want to do? For example, when the majority of students pass a written or oral test demonstrating knowledge of specific skills needed or complete a specific amount of work, they can move on to shop work, and the students who have not done so will not participate and will not receive the points? Perhaps clarifying where they are heading and what they need to do to get there will help the followers resist the leadership of those three seniors.
In addition, I recommend that you read through the other posts on this discussion board as they might trigger an idea you have not thought of before. I wish you well and hope that you will also receive suggestions from others with experience related to your field.