I've taught some network courses myself, and I think it's really useful to start with a general presentation of the different layers in a network stack, because lots of people are totally unaware of how protocols stack on top of each other and what are differences and relationships between "Internet Explorer", "HTTP", "TCP", "IP", "Ethernet" and so on; just don't stick too much to the ISO/OSI model, because, as everyone knows, it's more useful in a "general" sense than as an accurate model of how things work today.
My second step would be a THOROUGH dwelling into TCP/IP (with added UDP and ICMP, of course), because is one of the most misunderstood area for wannabe network admins; almost everybody can type and IP address in a network settings window, but a suprisingly low number of people know what it actually means and what a subnet mask and a routing table are; routing and NAT of course will be coming soon after. These are the most important topics of all: if you don't get them right, there's no point in talking about firewalls (I once met a guy who wanted me to run a firewall by simply connecting it to the network, because it just didn't know it has to sit between two places if it needs to filter the traffic between them; but that guy was probably more lacking common sense than network knowledge).
DNS is the obvjous next step, as you will of course need something a little more friendly to remember than those pesky numbers; this pretty much ends the "pure networking" sections, and if you want to share some historic knowledge you can of course drop some bits about things having been much, much more chaotic in the past, with different proprietary protocols and so on.
Next I would talk about the most common application protocols (HTTP/S, SMTP, FTP, etc.), without too many gory details, but with enough of them to make people understand why you just can't have two SSL web sites on the same IP address and port, or why e-mail was never designed to be 100% reliable and so you shouldn't start screaming if your message needs more than one minute to get delivered; and, of course, why FTP is the worse dream a firewall can have.
What comes next depends on the specific O.S. you're dealing with.
Windows administrators need of course to focus on Active Directory (and boy, there's PLENTY of it to talk about!), and then of course IIS; I'd leave out Exchange and SQL Server as they're pretty cumberstome beasts for a basic course, and I'd also leave out ISA Server, which is the firewall solution when Microsoft is involved, but can get quite complex to understand at times; if you need some routing/firewall capabilities to play with in a test lab, every Windows release since 2000 had built-in Routing and Remote Access which can do awesome things in simple setups.
I'm not really into Linux administration, but I'm sure other people around can give you some hints about that; IPTABLES of course is a must, and some Apache knowledge isn't probably going to hurt anyone.