There is a new feature in Google Sites that is going to make a lot of people happy! You can now add custom links into the navigation. You want a link to your SIS? You can do it at the top level now!
Now, when looking at the Pages window, instead of a "New page" icon, there is a plus sign:
Hovering your mouse over that + will pop up (currently) two options to choose from, the "New page" icon and a new "New link" icon:
Selecting the "New link" icon, you will get a pop-up window where you can give the website you wish to link to a name and copy in the URL. When linking, note that you can also select an existing page on your site (which will allow you to order the pages in the navigation menu differently, or even have a page linked in two places in the menu). You also have the choice of whether it opens in a new tab, or over top of the website itself:
You can then grab, drag and move the link as a main level or sub-level to any of your existing pages:
