This is the 741st most frequent Japanese word.
終わっ
"終わっ" is the past stem form of the verb "終わる," meaning "ended" or "finished."
In this sentence, '終わった' is the past tense form of '終わる' used to indicate the action of the class ending.
授業が終わったら、図書館に行きます。
After the class ends, I will go to the library.
Here, '終わった' is used as a descriptive adjective to refer to the work that has been finished.
終わった仕事を上司に報告してください。
Please report the completed work to your supervisor.
In this sentence, '終わって' is the te-form of '終わる,' used to link the ending of the vacation with the beginning of the semester.