What is the "studentd" process? I'm logging network traffic (with Radio Silence), and noticed that the studentd process from /usr/libexec/studentd is connecting to some server.

What is this process doing, why does it need internet access?
studentd manages the Apple Classroom experience for students and teachers that use MDM and the Apple School Manager service.

https://support.apple.com/guide/classroom/classroom-infrastructure-requirements-classes-apdd491bf924/mac
https://school.apple.com

The manual page also says it is designed to run without classes configured, so I would allow the traffic so that it knows you aren’t needing management and then sits idle.

https://eclecticlight.co/2019/07/24/studentd-and-its-self-signed-certificate-are-permanent-fixtures-in-mojave/

Unless there’s a resource issue, you have thousands of processes that run, self configure and then sleep or wait for a task. Blocking the traffic might make the daemon keep trying to connect and finish it’s run-once (or perhaps a regular check in) setup process.

On a larger stage, this design is how Unix and all of the Apple os work. They design small hardened daemons to serve narrow purposes, and often make a user account to run these behind the scenes to isolate permissions and sandbox everything cleanly. For us using the OS, I only try to manage these processes if there is a resource or usage issue.
In your case, this process will connect to very specific servers, so you can know it’s talking to Apple or to your MDM server in all likelihood. We would need details to dig into that deeper.