I have a client who thinks that there really needs to be a comma between the name of the school, and the public school zone number (CSD#). I would love a second opinion from a design perspective. Doesn't the contrasting weight and size of the CSD# separate it from the school name, making it unnecessary to place a comma or parenthesis in between? Any alternatives?
From a design perspective I agree with you that the contrasting text weight differentiates the two sections. To satisfy the client, rather than a comma which I think will look out of place, you can try a different character, such as this character "|" (I don't know what it is called but you find it on the \ key), or a dash or en-dash in front of the –CSD. Either of these will look better than a comma design-wise.
To answer your question, from a design perspective you've got it. Clear differentiation to me. From your client's perspective, they are likely coming from an academic background, or at some point they had a boss that insisted on doing things one way.
I run into this often with medical clients on punctuation. My solution for not sacrificing a design is to quote a contradictory source. Here's a link citing to NOT use punctuation for medical terms, yet I know the defacto Associated Press (AP) Style Guide clearly calls for it (my copywriters tell me all the time!!).
WHAT I FIND WORKS BEST, is to specifically inform your client of this gray area and why your design works better. This is important because they often will have to get your work approved in a conversation you likely won't be present in. If they can't (or won't care to) defend your point of view, they'll just come back to you saying, well my boss says to do X, so that's what we're doing. And then you'll have a piece that you don't want in your portfolio...
I like the idea of using the | character, but not so much the em or en dashes because they will just emphasize the different sizes of the two character types.
You could also use parenthesis around the CSD number in the same typeface. Graphically, they won't stand out too much, and will likely suggest the same relationship between the school and district that your client is trying to achieve by including a comma.
I think the difference in font formatting could work, but yes a client might expect something more obvious. Why not use larger pins instead of dots and put the CSD inside? Make both these and the text in a stronger color, not necessarily black. This way you don't need to add extra characters to separate what looks like too much text already, and you will construct a more infographic-style map.