I was taught in school that учень is pupil, but nowadays I see that it becomes obsolete
In Britain, pupil is still used to refer to a child of school age. Student is for higher education.
Should be, but depends on the variation of English you speak. Do I understand it correctly that AmE prefers "students"?
Yes. Pupil is not an unfamiliar word, but I can't remember the last time I used it. And I'm a teacher.
I think so too!
People often say "little brother" to mean "younger brother." It's not even necessarily a term of endearment, but is often just used matter-of-factly in lieu of "younger brother." Similarly, "big brother" is often used to mean "older brother."
Not sure if they're worth adding, but "little brother" comes to me more naturally than "younger brother."