Having been an active TESser (no spelling mistake, I assure you) and Edugeek lurker for some considerable time, I find myself in need of opinions.
We've been using Moodle for a long time now (anyone know Darren Smith/Dagza?) and we have gone through phases of strong forum use. We set up a 'subject support forum' which extended itself into an 'off-topic chit-chat' forum and we have a couple of other areas (such as a gamemaker course) which have a relatviely strong, but underground core.
I'm toying with the idea of setting up a separate forum site on the webserver and trying to promote one central place with a variety of boards in the hope of making it more overt and less of a nerdy dungeon (which is what it has become tbh).
On the other hand, I'm not sure if ANOTHER site is what the kids really need (they already have a website, a VLE, a blog site, a podcast site...). And if we do go for a forum - which software? vBulletin? phpBB? SMF? (I have mod status on an SMF forum elsewhere so am at least partly familiar with that). I would need LDAP support, and obviously have Apache, MySQL, PHP...
I did exactly that about 5 years ago at a previous school - was a great success - Departmental forums, year group, etc. for staff, subject, projects, general help for the students. Was heavily used and they really brought into the idea.
However, at my next school, they weren't interested. As yours seem eager for it, I'd certainly give it to them - anything to increase the lines of communication and wider learning in the school has to be a good thing.
I'd go with Invision Power Board myself - clean interface, lightweight, and powerful. vBulletin as a second choice.
Single-sign-on will be a killer app for you by the sounds of it!
Sorry, I wasn't clear - all of the forums we are using right now ARE moodle forums.
I was thinking about centralising the forums into one place. As I said, I'm not sure if it is the best idea but it's worth a go I think.
I also forgot to mention tha LDAP is a must have, and free would be really helpful (although not necessarily critical). So far phpBB looks like a possibility and I've found an LDAP hack for SMF, although I am wary of hacks...
vBulletin is very very good (I should know )
Not sure about ldap but there will probably be a addon/workaround for it. Its worth trying to find a way to get vBulletin to integrate as it really is the best forum system I have used.
It handles load/users better then anything else and the who system is very developed.
I have used phpBB linked into postnuke with huge amounts of users in the past and it really couldnt cut it.