It must be understood that this is not "art". Some rigor, precision, and careful work is necessary. This is not to say that the student may not be expressive, but this expressiveness must be appropriate to the assignment. For example -in drafting, there is very little room to set aside proper drafting conventions. However, in sketching it is possible and sometimes desirable to develop a certain looseness, this is not to say sloppiness, but rather the result of efficient (quick) work.
I grade students first efforts at drawing (not drafting) as follows:
FLOOR PLANS Sketches:
It is important that things be roughly to scale. It is common for a student to get all wrapped up in detail and loose sight of making everything fit. For example, he, (or she) will draw every detail of their desk / computer / stereo, and not have enough room on the wall and they end up with a door that is 16 inches wide -at scale. Or the toilet is the size of a hot tub -or dinner plate. Ideally, the student will do a rough sketch to establish proportions, working lightly in pencil, and only then go back and do details.
I do not worry so much about straight or even lines, nor the level of detail. Erasures are a fact of life and a necessary part of the discovery process. Part of my grading is a consequence of looking at (and drawing) a lot of floor plans, and I while I've never seen the student's home, I can recognize impossibilities or unlikeness in the student's work.
ISOMERIC DRAWING:
The nice thing about this assignment is that it is largely self grading. If the student does it wrong, it is immediately and abundantly clear it's wrong. Those students the "can't draw" can draw if they follow the rules, -vertical things are vertical on their paper and parallel things are parallel. Following these rules will lead to an acceptable grade, (and often some pleasant surprise on the part of the students that "can't ..." ). One ambitious student will redraw his or her work and it will be meticulous in it's craftsmanship, if a bit unimaginative. Another will push the envelope and try to apply the rules to different shapes and sometimes make wonderful discoveries on their own. I try to have both these opposites in mind when I grade papers.
PERSPECTIVE DRAWING:
Much like isometric drawing, but with more complex rules, which, if followed, lead the student to impress themselves. I grade based on following these rules and careful workmanship -particularly important in perspective drawing. I try to be aware of the student that does meticulous but unimaginative work as well as the student that pushed the envelope.
RENDERING:
Same thing, but more of it. The student either successfully follows the rules, or successfully pushes the envelope. "Loose" is acceptable -provided it is a consequence of experimentation, sloppiness is not. A good rendering is the most profoundly self grading and success is most self evident. If the drawing looks like something, (presumably what the student wanted it to look like, -but there is no telling), then the student has suceeded.
In any of the above, you should be able to see why I gave a given student's work the grade I did.