First of all, it is a good point that every Stack Exchange site includes a short blurb describing the target audience. Here are a few:
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields.
MathOverflow is a question and answer site for professional mathematicians
Mathematica Stack exchange is a question and answer site for users of Wolfram Mathematica.
Academia is a question and answer site for academics of all levels.
It seems to me that a good one for this site would be something like:
[Name to be determined] is a question and answer site for mathematics teachers, math education researchers, and anyone interested in the process of teaching or learning mathematics.
It's true that it feels premature to talk about this sentence before deciding on a title. On the other hand, I discussing this sentence might be helpful for choosing a title, because it clarifies the different ways that we all think about this potential site.
It seems that one main point of contention is whether we want students on this site. My answer to this question is yes, but only if they want to talk about teaching, curriculum, or pedagogy. I can certainly imagine student questions that would pertain to these topics, such as:
Why do we learn straightedge and compass constructions in geometry class?
Why is it important for me to learn to do math without a calculator?
Why are real analysis and abstract algebra required for a math major?
Is there a good way to remember all of these logarithm rules?
Is it better to take AP Calculus or AP Statistics?
How can I study for calculus tests more effectively?
(Note: If you like one of these questions, feel free to post it as an example question on the main site.)
At the same time, I think it's much more common for students to have questions about math itself, as opposed to the process of teaching or learning mathematics. Any question that's primarily about mathematics should be migrated to the Mathematics Stack Exchange, and it's an important practical goal to minimize the frequency of such questions. As I have stated elsewhere I think this means that the title of the site should be very clear about the type of questions we're looking for.
In summary, I don't think that we should exclude students (or any interested audience), but I do think it's very important to exclude certain types of questions.