By default Google Sites are set as private under our school domain. In order for your advisor (and anyone else that you want to share the site to), you need to change the sharing permissions. Once this is done once, you won't have to worry about it again.
NOTE: Just because your site is set to ANYONE WITH THE LINK, this does not mean artifacts/projects that you put on the site will also have that sharing permission. Anything that you link or embed must have a sharing permission of anyone with the link.