"Kelas itu selesai."
Terjemahan:The class finished.
They should! "The class is over" is the same. Let's report it. "The class is over" is not a prepositional phrase, it's a normal sentence, it has a subject, a verb, and a complement (here: adjective).
The only difference is that "finish(es)" is active, and "is over" or "is finished" is passive.