der Gymnasiast
Gymnasium student
A student who attends a Gymnasium. The female form is die Gymnasiastin.
Als Gymnasiast musste er viel Latein und Mathematik lernen.
As a Gymnasium student, he had to learn a lot of Latin and mathematics.

Gymnasium student
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Gymnasium student
A student who attends a Gymnasium. The female form is die Gymnasiastin.
Als Gymnasiast musste er viel Latein und Mathematik lernen.
As a Gymnasium student, he had to learn a lot of Latin and mathematics.

The word 'Gymnasiast' (Gymnasium student) is derived directly from the word 'das Gymnasium' (academic secondary school). It refers to a person who attends this particular type of secondary school.
Mein Bruder ist Gymnasiast in der zehnten Klasse.
My brother is a Gymnasium student in tenth grade.
This term is specific to the education system in German-speaking countries. It is used to clearly distinguish students from those at other types of schools, such as the Realschule or Hauptschule.
Die Anforderungen an Gymnasiasten sind oft höher als an andere Schüler.
The demands placed on Gymnasium students are often higher than those placed on other students.
The word is often used in the construction 'als Gymnasiast' (as a Gymnasium student) to describe a role or a period in life. Typical verbs are 'lernen' (to learn/study), 'müssen' (must/have to), or 'sein' (to be).
Als Gymnasiastin hatte ich fünfmal pro Woche Sportunterricht.
As a Gymnasium student, I had physical education five times a week.
student / pupil
'der Schüler' and 'der Gymnasiast' can both refer to a person at a school. 'der Schüler' is the general term and does not yet say which type of school is meant. 'der Gymnasiast' is more specific: he attends a Gymnasium. The words can be confused because a Gymnasiast is always also a Schüler. If the type of school is important, use 'Gymnasiast'. If you are only talking about school in general, 'Schüler' is more natural.

Im Zug liest der Gymnasiast ein Buch für den Deutschunterricht.
On the train, the Gymnasium student reads a book for German class.
Für das Projekt sucht Selma einen Schüler aus der neunten Klasse.
For the project, Selma is looking for a student from ninth grade.