Single Sign On - what is best

Hello

currently looking at Moodle 2.4.3. On a Windows 2008 R2 server with IIS 7.5. and a DMZ.

The Moodle is available both inside a college network and outside the college network.

We currently use LDAP to authenticate users. They have to log into the Moodle using their network credentials.

We want to enable SIngle Sign on inside college whilst still having the home page of the Moodle visisble outside of college.

Students and staff currently log college computers onto the newtowrk using AD.

What method of attack would you use to get SSO working inside college whilest leaving the home page visible outside.

Thanks

Heather