microsoft AD is several well integrated things in one. But in this FOSS world you get all the building blocks, but you may have to assemble them yourself..
Thus you get great flexibitity, and power. But it may be that you are not very interested in tinkering with the internals. And building your enviroment from the ground up.
You should atlest test out the Debian Edu /Skolelinux [1] distribution, It's made to be easy to admin for a part time teacher /part time admin. And comes with openldap+bind+saba+winbind out of the box.
You can easily join linux, mac, and windows machines into the domain. have central authentication, and roaming profiles. And is tailor made just for your use case.

Debian Edu Homepage http://skolelinux.no/en/
Debian Edu Wiki http://wiki.debian.org/DebianEdu

Ronny Aasen

DISCLAIMER :: No knowledge of the quality of the book. ::
My history of electonics (offhand translation to english) book that i read in school some years ago. Claimed the main reason for betamax failing, was the rental market. Since the first models of betamax players rewinded the tape without unloading it from the readhead it wore out the tapes a lot faster then vhs.
naturally videostores choose vhs that lasted longer. even with it's inferior quality.