Studentene øver på presentasjonen i klasserommet.

We’re talking about the students (a specific group), not “students” in general. In Norwegian you form the definite plural by adding the suffix -ene to the indefinite plural noun:
• student (a student)
• studenter (students)
• studentene (the students)

på is the preposition that introduces what you are practicing.
• øve alone can mean “practice” in a general sense.
• To say “practice something,” you use øve på + object.
Example: øve på presentasjonen = “practice the presentation.”

Because we refer to the presentation they’re working on, not just any presentation. Norwegian uses a suffix for the definite singular feminine noun:
• presentasjon (a presentation)
• presentasjonen (the presentation)

In Norwegian, when you’re inside an enclosed space you use i + definite form:
• i klasserommet = in the classroom
Using på with klasserom would sound odd, because på is used for surfaces or open areas (e.g. på bordet, on the table).

Klasserom is a neuter noun meaning “a classroom.” To make it definite singular, add -et:
• klasserom (a classroom)
• klasserommet (the classroom)

You change øver to its past form øvde (regular weak verb):
• Studentene øvde på presentasjonen i klasserommet.

• øve = practice skills, drills, rehearsals (e.g. a speech, an instrument)
• trene = train (often physical training or sports)
Example: You øver på presentasjonen, but you trener på fotball (train for football).