No, they're not interchangeable.
"In Germany, the German cognate term "student" is reserved for those attending a university. University students in their first year are colloquially called Ersties ("firsties"). Different terms for school students exist, depending on which kind of school is attended by the student. The general term for a person going to school is Schüler or Schülerin. Students attending a university preparatory school are called Gymnasiasten, while those attending other schools are called Hauptschüler or Realschüler. Students who graduate with the Abitur are called Abiturienten."
At the beginning of a word, yes. In other positions, it's a bit more complex. Have a look at this: