Many people assume that as they are websites, they must automatically be 'public' but this is not the case.
The reason why people outside of UWCSEA can't see your site is because the share level (permissions) of your site is set to only people within the college domain, ie people with a @gapps.uwcsea.edu.sg address, which is the default setting for all college google sites.
This means people, eg parents, can only view the site, via their child's log in - this is how I believe most teachers manage this.
To make the site viewable by people from outside the college domain (eg parents) you will need to change the site's share level (permissions). Only the owner of the Google Site can make these changes, - (by the way you can have more than one owner, although there is not much more they can do, except change site permissions).
So click on the 'Share' button in the top right corner, then you can change it to 'anyone with the link' or Public.
The problem then is that none of your students will be able to EDIT the site, only VIEW, to get around that you need to add them individually to your site permissions, where it says 'Add people:' these people you have specifically added will be able to EDIT the site.
You should be able to easily add your class using this list, eg: "Grade 15 Class SMc Dover" <firstname.lastname@example.org>,
It is worth noting that the view status will not allow ANY editing, it really is an 'all or nothing' scenario for now - not even posting comments is allowed unless you can edit the site. If you attempt to add people from outside of the domain, they will need their own private Google Account to be able to accept your invitation and edit.
Finally ... now your whole site is viewable by anyone with the link - if there is a section of the site you do NOT want to be viewable there is a feature (page level permissions) that allows you to control specific pages, but only at a level that is more restrictive than the entire site, like making some sections private, but that does get a little convoluted and complicated, you have been warned!