Hello and good luck with your studies.
Most small to mid-sized businesses will have more a generalist or two who over see all of the server roles.
In most of these situations you will need to know:
Active Directory with Group Policy
- How and why to make new OUs.
- How to make and apply group policies especially for
-- password complexity
-- folder redirection
-- various Internet Explorer settings
-- server security
[*] - How to make effective groups both for the business management and for managing network access (they aren't always the same)
[*] - Understanding replication
[*] - Sites
DNS -- you can't have AD without it.
- issues of split DNS vs single DNS
- AD integrated vs separate DNS
- forwarding
- replication
- adding hosts to external DNS
- mx priority
DHCP
- basic configuration is sufficient for most small/med businesses. Understand how DHCP works, APIPA, and how reservations work.
- larger networks may need DHCP forwarding/relay or multiple DHCP servers and scopes.
File Services
- Know how to create shares and properly apply both Share and NTFS permissions and how the two interact
- how to map drives through command line, scripts, group policy
Print Services
- How to install and share a printer (includes permissions)
- How to map printers to clients
- how to upgrade drivers on printers and clients
- how to migrate printers to a new server
Those are the very basics that just about every company that has a Windows AD domain is going to be using. Web servers are also very common. Terminal servers a bit less so, but still common. If you get all of the above down, you'll have a good idea of what else you want or need to know.