Sharing a folder on server

There is no ease way to set this up in this situation. And for the growth of use of the school district's network in the future, you're better off set it up correctly now. First you should have a WIN server and DNS server setup ( if you haven't haveit setup yet ), then you'll have to set up the Trust Relationship between each domain, after you've done that, select a server that you're going to have the shared folder created, create the folder and make sure you shared that folder and then go into that folder's properties, permission, and add those names that's allowed to access the folder from each domain, and also assign proper permission.
Those that have given permission to the folder should be able to access the folder using 98 machines. You just have to show them how to browse through the network using network neighborhood and get to the server's folder.
Hope this helps.

Here's a little more help.
So far Answer 2 is closest to the mark.
Assuming the Macs are already able to communicate within the domains in which they exist, nothing further needs to be done there (except the actual drive mapping one you have the shares set up).
If you have Macs existing in the domain belonging to the server where you install the shared folders, you probably already have Services for Macintosh installed. If this isn't the case, you need to install it.
The only other way around all this (creating trusts, etc.) is to create user accounts for all users needing access to this shared folder, on the DC of the domain where the share exist. If you take this route, to make life simpler for everyone involved, make sure the user ID and password are the same as what the users are using to log on to their current domains. This allows you to use "pass-though" authentication to access the shared resources.
As for Answer 3, you only need ipx/spx if you are connecting with a Novell network. You only need Appletalk if your Macs are connected via a Localtalk network.
Hope this helps.