Hi
You should definitely focus on HTML/CSS rather than WYSIWYG software... but I think web design means more visual design, interface design, and information architecture... for that you need PhotoShop or similar software... but also for a successful website you need to have solid knowledge of html/css, to know what can be coded and what not...
The only software I personally use for coding because of "code sugestions" feature is PHP Designer, which is mainly a PHP/MySQL programming software, but also has support for JavaScript, HTML, CSS and other technologies... and they have Academic Licenses which costs only $32...
Hope I was helpful, Good Luck with teaching :)
Cheers!
Alex Mos - Web Designer / Founder
http://www.WebsonDesign.com