It feels like the latter is more specific...
Which one should I use when addressing a high-school student?
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"Ученица" is either a school pupil or an attendee of extra-curriculum studies, e.g. a language course or a subject taught by a private teacher, and "студентка" is a university student only, meaning that the two cannot be interchanged. Therefore, a professor would say "мои студентки", but a school teacher "мои ученицы". Actually, "ученица" is simply anyone whom one trains or explains material to.
Before 1917, there was also the term "институтка", used to refer to students of Russian female high schools.
Ученица is a girl who studies at school. Almost all of them are 6-17 years old.
Студентка is a girl/woman who studies in higher education institution. Most of them are 17-22 years old, some are older.
For техникум, both terms are used. Sometimes, students under 18 are called ученики/ученицы, while students 18 or older are called студенты/студентки. But the most common term for an adult who studis neither in school nor in a higher education institution is учащийся/учащаяся. For military courses, a student is called курсант.
ученик/ца is anyone who's formally learning/studying something, who's
учится, either a theory or a skill
in modern Russian culture addressing a person using terms designating their occupation isn't normally practised (an exception would be addressing a driver of a regular bus or a minibus, the proverbial and the notorious
thus whether she's a schoolgirl, a student or anyone else, a young woman is normally addressed with
if you're her coeval more or less, a very informal and a kind of playful way would be
addressing her from a position of a much older person,
Девочка! can be used
incidentally and unbeknownst to me the author had earlier already inquired regarding the exact same set of terms