Web Design is exactly what it says; the designing of the website. When you go into writing code you're talking about Web Development.
The first bit of advise I can give you is to change the name of the course to Web Design & Development. The second part would be to drop all talk of SEO and Marketing because it is out of scope. If you want to teach HS students how to build a website then I would limit it to design and development, and to let them become good at that first.
In other words, I'd make the first level contain:
- HTML from the ground up. Teach a bit of HTML5, but make sure you set the foundations of HTML up and exactly what HTML is. The main thing you should be teaching is about structured data, not how to build a cool website.
- Teach a bit of CSS to get them going. To make things fun I would probably limit them to the newest versions of Firefox and Chrome, because HS students will HATE debugging IE issues.
- Spend at least half of your time showing users how to design a site, preferably in Photoshop
- NO DREAMWEAVER!
For starters, you're touching on security without any mention of server-side coding, so you're not really covering much in terms of security. Putting a plaster on a wound doesn't make you a surgeon.
The first level should drum home as much HTML as they'll need to know, so give them a small introduction to HTML5 and some more CSS and then leave them to learn whatever else they'll need to make a site.
You've listed WordPress as a CMS, which is a bad move as WordPress is a blog script that is used by entry-level developers as a CMS, usually because they have no experience of a real CMS. If I were you I'd install a copy of Concrete5 or Drupal and just show the kids what a CMS can do. It's all you can do without teaching them about server-side coding...
Again, do NOT teach the kids Dreamweaver. Even kids of that age will know that no one uses Dreamweaver any more and your course will lose all credibility.
If I were you I'd use the second level course to get the kids working on a project. A good task would be to get them to write the front-end of a blog together. Give each kid a task to do on the blog and get them to agree on an overall design to work from and then give them that time to build the design and front-end. By the time the course is finished you can plug it into some back-end code you've written and they can have their own blog, leaving your final lesson to show off how it works and how what they've created is largely the kind of stuff they may be doing in industry.