Given the load of a cart of MacBooks on one or two access points, you will need to seriously consider using portable home directories for these machines. This will allow you to use centralized authentication and authorization, but will dramatically speed up logins and if setup properly will even make applications like Office 2004 run fairly smoothly. The trick is doing the testing to determine exactly what stuff should be synchronized on login and logout and what should only stay local to the laptop. Though not required, it will make life easy if you also assign students to laptops with this setup. That way, Susie Q always logs into the same laptop where her portable home directory has stored all her preferences locally. It will reduce the data transfer during login and logout and the users will always get the same set of preferences.
The other option is to setup a generic local login account on each of the MacBooks and then have the users manually connect to the sharepoint where their home directories are. Again, little to no load on the access point,but it's an extra username/password for the students to remember and an extra step to get to their home directory if they need to save files centrally.
Acceptable data: It's also worth clarifying what data is supported for backup and what is not. With the proliferation of iTunes & iPhoto users end up with gigs of personal data on the laptops. I'm not sure that this data should end up on the school's home directory server so some work must be done ahead of time to keep that data from being synchronized from laptop to server. Also it must be made clear what data will be backed up on the server and what data the user are responsible for. It's worth having a plan for backup of documents and that should be in writing. With roaming profiles/synchronized (Windows) and portable home directories (Mac) it's pretty easy to handle centralized data backup, but the users need to understand that they need to use their computers on the school network for those backups to be successful.