There clearly is something wrong with name resolution if you can't even ping the server by name. If your clients can't resolve the name of the server to an IP, then we can't even think about accessing a share in the form of \\servername\sharename.
However, you CAN test \\serverIP\sharename and see if that connects. Meaning, if your server has an IP of 192.168.50.11, then from a client, try to connect to \\192.168.50.11\sharename. If that connects, then your file sharing is OK, and you just need to fix name resolution of the server.
You are correct in that you do NOT have to put the d$ share in the UNC path; should just be \\servername\sharename or \\serverIP\sharename
Seems to me, that your server still thinks is has an FQDN of SCHOOL.tech.schoolname.org. So are you telling us that you've removed the DC role from the server, and have placed the server back into a WORKGROUP, using the same workgroup name as the clients. Confirm ALL of the TCP/IP properties on the server -- did you set the DNS server(s) back to what they were pre-AD (pre-AD sounds weird)? Are you using WINS in your org and did you check the WINS settings? Did you check if NETBIOS is enabled on the server?
I'll just briefly respond to a couple of points in your last post relative to moving forward with the AD implementation.
It is a good practice to name your AD domain name something like schoolname.LOCAL rather than using your public FQDN (SCHOOL.tech.schoolname.org). There's nothing stopping one from using their public domain name as their AD name, but as you can imagine there are considerations that need to be made for accessing public servers of the domain from internal clients. So make it easy on yourself and don't do it.
You were correct in defining DNS Forwarders in your first attempt, but missed a couple of other steps which we can talk about later.
You won't host your public DNS on the AD-controller (DC), so don't worry about that. DNS is used on the DC's to store AD-related information and to do name resolution for the organization, and is not necessarily intended to be used as your public DNS host.
At least on Windows XP it does.