OK, So what you need is their external facing site.
It wil be something like
https://webmail.organisationsite.com/owa or something to that effect.
If they do indeed use exchange for their mail, and not some other hosted substitute, you will be able to add it as an exchange account.
Make sure you add the IP of the external facing site as an external server, use the URL as internal (this should allow you to use the services inside the school network).
Any problems, PM me and I can see what I can do to help you.
EDIT - I see you posted the link...
So for the IP part, put in 68.164.103.173
Your username should be entered as it would on the site, or sometimes, you might need the domain.
This is found under your login screen when you login at school... might be something like;
Username - Your UID
PW - Your PW
Log onto - Domain Name
You may nee the fully qualified domain, which would be shoshanascardin.org