There is nothing wrong with homework an sich. I have never understood why anyone would want to treat homework questions different from 'other' questions.
Many homework questions actually already meet the criteria for another close reason; mostly too broad or primarily opinion based.
There are some homework questions that don't show any effort, but aren't strictly speaking off topic. In my opinion, we should just downvote them. New users will understand that that's not the way to ask a question. However, if someone has the time and energy to write an answer, why not? It is not that the question is unanswerable within our scope that we should close it.
As I see it, closure is a method to prevent Evil from spreading through the site (i.e., to prevent to get bad answers because of one bad question). It is not primarily a method to show to new users what our scope is - that should be clear from the help center, and more importantly perhaps vote counts.
Creating a homework tag is a bad idea. Some years ago already, Jeff Atwood wrote a blog post on "meta tags" (subjective, best-practices, beginner). Let me quote a bit of something he quoted:
There's been a major uptick recently in tags that are not useful and just add noise. [...] this particular category of tags is one that has been historically referred to as meta-tags on MSO, and these tags cause a lot of problems.
The reason meta-tags are a problem is that they do not describe the content of the question. They describe some other aspect of the question, like the author's skill level, or the author's motivation for asking it, or generally what "kind" of question it is (poll, how-to, etc.).
Meta-tags are actually a subset of a larger problem that I usually call dependent tags. These are tags that don't say anything by themselves - you can't tell what the question is about unless they're paired with some other tag (or several of them). These tags are a problem because people don't realize this and will often use that as the question's only tag.
And since then, meta-tagging is 'explicitly discouraged'.
I have seen meta-tags in practice on EE.SE some time ago, and you really don't want to go down that road. People use "homework", "beginner", etc. as the only tag, which makes the site so much harder to search. Furthermore, these tags are redundant, because they don't say anything about the question itself (rarely will it happen that someone adds the "homework" tag to his list of favourite tags).
A separate point would be if we should have a homework policy and what should be in it. I think this would be a great idea, but something for a separate question. The main point should be (as has been mentioned in another answer here already) to never give a direct answer, rather give hints and help the OP to find the answer himself.
On the other hand, if people are going to commit plagiarism that isn't our responsibility. Almost all my homework for university is publicly available, also before deadlines. What people do with it is their business. My website is searchable, so if they hand in the exact same thing, a teacher (or automated plagiarism checker) will find it with basic searching skills.