Plone seems to be the fullest featured 'heavyweight' CMS out there. CMSs are very variable beasts - from an application to make a pretty website (eg Typo3), via Joomla, Drupal, Xoops etc etc, to 'do everything groupware' (eg eGroupware).
Plone is nearer the latter and is our choice to fulful many of the latest Becta requirements. I'm just about to install at school after trialing at home.
It authenticates via LDAP and thus every teacher or student can have their own e-portfolio. It understands active directory groups and so each group can have its own shared area and every user can share stuff if they want. There is the usual CMS document management/review/publish stuff and a wealth of addins.
Setting it up is challenging! Linux server, Python, LDAP, Zope, Plone, Apache (because it's web server MUST be proxied) and then configuring it all, but it works beautifully and has a clean interface.
I've been researching this stuff for months and for organising the huge amount of documentation that a school needs - it's the best as far as I am concerned.
If anyone wants to know anything else - let me know!
Cheers
Kevin