I think its the other way around - he has to take at least one class, but can also take both. There's nothing that implies he has to take both, but the word "or", in the context of that sentence, stipulates that he has to take at least one. Leaves it open to whether he can take two

Another way I'm thinking of this to help me understand the Kumar situation is like this:

One can be considered a cat only if they are a part of the mammal animal family. Bob is a mammal.

Does this mean Bob is a cat? No, not necessarily. Because he's a mammal, he has a chance at being a cat, but we don't know whether he is a cat or not. We just know that because Bob has fulfilled the necessary condition (being a mammal), he is in the running for maybe being a cat (sufficient condition). He could be a dog, human, rabbit, etc. Who knows. All we know is that Bob is a mammal, so there's a chance that he could be a cat. If Bob was a chair, there's no chance he could be a cat - because he hasn't fulfilled the necessary condition of being a mammal.

Now let's apply this to Kumar.

Students can be* cited as "late" only if they arrive more than five minutes past the last ring of the homeroom bell. Kumar arrived 17 minutes late.

Does this mean Kumar is cited as late? No, not necessarily. Because he arrived 17 minutes late, he has a chance at being cited late, but we don't know whether he has been cited late or not. We just know that because Kumar has fulfilled the necessary condition (arriving more than 5 min. late), he is in the running for maybe being cited late (sufficient condition). Say he arrived 16 min late, 15 min late, 14 min late, etc. - as long as he arrived more than 5 minutes late, there's a chance he could be cited.

*changed the original "are" to "can be" for clarity

Let me know if I've got this wrong, but thinking of it this way made the official explanation make more sense to me.

This was really helpful, thank you!