Is a question off-topic, just because it is asked by a student?
No, certainly not.
I believe the ban of certification and education questions was meant to cover questions about the content, usefulness, difficulty, etc of particular certifications or degrees. I believe it was not intended to ban technical or theoretical questions that someone might have when he is studying for a course or for certification.
You would be wrong. This has been covered on meta before multiple times, here for one of the earliest examples on meta, although there were other discussions before
IMO, a question should not be banned however, simply because the person asking came up with the question during his education/certification; for example while writing a report, cramming for an exam, listening to a teacher in class, reading the assigned material for a class.
I think we all would agree that closing a question simply because the person came up with the question during such activities isn't appropriate...if they are asking about the concept/theory to understand what they "came across" during the activity and can provide a good question (not too broad, not opinion based, etc).
To most of us, this is no different than when the server person in the IT department is tasked to upgrade the xxx on the network and has a question about what they are doing.
However, the actual questions from homework, certification, testing, interviews, and so forth are not considered on topic and they should not be.
Homework/certification questions are often contrived learning situations that have very little application to the real world. Their intention is often not to provide a real world solution to how something is done, but rather to force a student into learning the concepts and theory.
Subnetting questions are one of the easiest examples to come up with as they occur so often. In the real world, a professional isn't going to create a /27 subnet for that group of 23 computers/users as that makes no sense (no room for growth, changes, etc). They also aren't going to carve up a /24 block into seven various sized blocks to accommodate all those little subnets for user devices in the vast majority of situations.
These types of questions also may require specific answers that are applicable only to the context of the question. One example is "how many layers in the TCP/IP network model"? Depending on the context, the answer is either four or five (Cisco in particular always refers to it as five for it's testing/certification).
Or these types of questions may include historical trivia or background for reasons of teaching that don't apply to modern networking in any sense. The prime example here (as regular visitors will likely know) is classful networking. This is still taught quite heavily, but classful networking is long dead and will never return. References to it do not apply in any way to networking today.
While there are many reasons for not allowing these types of questions, I will only give one more. Our community is filled with professionals that work on networks daily. These types of questions are often ones that are not "interesting" to these users in any real sense and are often asked by visitors that do not become regular members of this community. If the community caters to questions that do not provide value to our regular members and continue to bring them back, they will choose to spend their time elsewhere. This community will no longer exist as it was envisioned/built by those members.