1. Even one or two zeros has a really devastating effect on a student's grade, especially in a traditional 90-80-70-60 scale, even worse if the people are using one of those idiotic "honors" 94-88-80-75-65 scales. (These scales are idiotic, because performance on more complex tasks--the kinds you'd want students to be doing in honors courses--is harder to replicate, so you should if anything have a looser grading scale that allows kids to show excellence sporadically and still get good grades. For example, classes for math majors at the college level not-infrequently use the "sup norm": your grade is primarily based on the highest score you get on a test, rather than the average; lower scores are basically ignored. But I digress.) For example, a student with a consistent 90% average on 9 assignments (low A) drops to a low B with a single zero, and even if the student gets 100's on every assignment thereafter, it will take that student 9 consecutive 100's to bring that grade back up to an A. Put differently, on the 90-80-70-60 scale, a kid needs to have 95's on EIGHTEEN assignments to get an A if he or she gets a single zero.
It's depressing that so few teachers using this system understand this. I mean, are they blind?
2. Students DO understand this, and so after a couple of zeros, they correctly conclude that there's really no point in trying anymore. This outcome is bad for everyone, because it means that the students stop making any attempt to learn any of the material, and just sit around disrupting your class. You as a teacher lose both your carrot and your stick.
3. This system, when applied to homework, is even stupider, because the only point of homework is to (a) develop the ability to work independently and (b) develop knowledge of the material. If the student is not doing homework, a system that very quickly tells him or her not to bother to do any more homework is obviously not going to develop his or her ability to work independently, and is obviously not developing his or her knowledge of the material.
4. In fact, if you think about why students might skip doing homework, I consistently hear three reasons. (i) I can't do it or don't see the point; (ii) I can do it easily and so I don't see the point; (iii) I have too much other homework to do. In case (i) , the student is telling us that he/she can't actually work independently on the assignment, or that there's no obvious reason to do so. So a zero penalizes the child for not doing something he or she perceives as either undoable or pointless. In case (ii), the child is saying that while he or she could work independently, the assignment is not actually going to develop his or her knowledge of the material. So giving a zero feels to me like you're mad at the child for uncovering the secret that your assignments are actually irrelevant to their learning process, whereas instead you should be saying something like "good metacognition; what was something interesting you were thinking about?" In case (iii), the issue is not that the kid doesn't recognize the importance of the assignment, but that he or she has too much other stuff to do to actually get done this thing that he/she agrees is important. So giving a zero penalizes the child for a problem not of his or her own creation, and frankly, makes you part of that problem instead of being part of that solution. (Note that your work has been given a lower priority than other work, usually--in my experience--other work that the student felt more relevant to his/her learning, or more within his/her grasp, or with more obvious positive/negative consequences.)
5. Therefore, you as a teacher should do everything in your power to avoid students getting zeros: give them second chances to do assignments, make them up in front of you, show proficiency in alternative ways, etc. In particular, you should avoid as much as possible policies that give automatic zeros for any but the worst behaviors.
6. Finally, giving an automatic zero for a late assignment is just stupid, because what you're telling the kid is that doing the work one single day after it was due is totally useless. But what kind of teacher assigns work that is meaningless past a 24-hour expiration date? This is supposed to be cognitive development, not milk left out on the counter. Of course, there are occasionally assignments that really need to get done by a specific time in order to set something up for class. But then that can be communicated directly, outside of the code of grades. "I really need you to do those coin flips tonight, because tomorrow we're going to aggregate our data." "I really need you to practice these derivatives tonight, because tomorrow we're going to work on applications." "It's really important that you do the assigned reading every night, because otherwise you'll have nothing much to say in our discussion of the texts the next day, and you won't even really understand what the rest of us are arguing about." And then you make the assignments short and meaningful; in the last case, for example, you can assign a short response paper rather than an outline or ...
If the homework is essentially skills practice, and the skills are important, then the kid will still be well-served by doing the practice a day or two later. In fact, if you ask more questions instead of giving the kid the zero, you might find out that the kid didn't feel intellectually able to tackle the material when he/she got home: you're penalizing the kid for being a slow learner.
7. Finally finally, it's important to remember how much relying on work done at home for learning privileges kids who are already privileged: kids who don't have to work to help their families pay rent, kids who don't have to watch small children (brothers/sisters/cousins) so that other family members can work to pay rent, kids who have a quiet and reasonably conflict-free space in which to work, kids who have parents or other family members whom they can ask for help, kids who have consistent access to the internet or other non-parental sources of instructional support. Yes, it's important that kids do learn to do work outside a supervised environment, and yes, it's often difficult if not impossible to get everything covered and practiced in the time allotted. But remember that every time you rely on homework as a part of the learning process, you're giving more advantages to the kids who already have the most, and throwing up another barrier to the success of disadvantaged kids, which is really the opposite of what public school is supposed to be about.
OK. That's off my chest. But I'll sign off with one last h/t to Sean, who is really awesome. When asked by an audience member why we should assign grades in a way that allows kids multiple opportunities when "in real life, you have to get it right the first time," Sean gave the courageous--and totally true answer--that in real life, you almost never have to get it right the first time. Most of us have made LOTS AND LOTS of mistakes in our jobs without getting fired--often,without being yelled at. And even the "exceptions" Sean cited--surgeons and airplane pilots--are not really exceptions: they just practiced, under supervision, in training, getting it wrong lots and lots of times in simulations (sewing cadavers, practicing takeoffs in a flight simulator or with someone else sharing controls) until their accuracy rate improved to an acceptable value for "real life." This is learning, peoples, not the Spanish Inquisition.