Having said all that, the "lab report" is not a bad thing.
Yes, it is artificial. Yes, it is a weird chimera. But some of that weirdness is necessary for pedagogical reasons.
We can't expect students to write professional-grade research papers on the first try.
My recommendation is to treat the "lab report" as a watered down research paper. (FWIW I think of the traditional "science fair" poster in mostly the same terms, i.e. as a watered-down research paper.)
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I agree with this, but I will add another aspect that I think makes the lab report even more important.
We often hear the adage, "You don't really understand something until you have to teach it." I'll reword that as, "You don't really understand what you've done until you have to describe it to someone else."
Most students who can't write a decent lab report also cannot sit down in my office and explain to me verbally what they did and/or what it all means. There are always exceptions... there are students who can explain things face-to-face who don't seem to have the ability to write it down.
Work sheets don't involve the same thinking skills as explaining something to someone else, and worksheets are also easier to copy or mimic. Putting into words what you did, why you did it, and what it all means, can be an important part of learning the science. Most often, a student who cannot write a reasonable report also does not understand the experiment. Indeed I have had students who can discuss the experiment with me and convince me they understand the experiment and the science behind it, yet they just can't get it written down on paper. I spend a fair amount of time working with these students, so it's a good thing there aren't too many of them.
As for the specifics of things like passive or active voice... I don't care, unless (as Denker states) they switch back and forth. I also don't think it is appropriate to demand past tense, present tense, future tense, etc. because all are appropriate at certain times and certain places. When describing what the student did, it's appropriate to use past tense. When describing physics principals that reflect current understanding, it is appropriate to use present tense. When discussing future work that is planned to clear up some lingering questions, it is appropriate to use future tense.
Unfortunately, many students switch through various tenses and styles in a haphazard manner that seems void of any thought about how to write or how to "tell the story." For example... "In this experiment I will measure the half-life of 137m-Ba. We used a 137-Cs/137m-Ba 'cow' to get the 137m-Ba. Be sure to use the right solution for 'milking the cow'. We are counting the radioactivity with a GM-tube that we will connect to a scaler. We were sure to wear aprons and rubber gloves. I started the timer when I started the first count. We will take counts every two minutes for 16 minutes. We are not moving the source between counting periods. The first thing we did was make a count of background."
Notice that this hypothetical student at least stated some important physics ideas. The student also mixed in some extraneous sentences that don't really play an important role in the physics, and this makes me wonder if the student knows what is important and what is not. I would say that early in the term, 25 to 50 percent of my students write in this manner. It's as if they are sending me a stream of "tweets" as things that "might be important" pop into their minds. Knowing this represents many entering students, I can't let it go.
Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657