@Henning *********
be careful. If you already finished all your classes and exams, you are no longer studying, and therefor not eligible for the student visa extension anymore. As soon as you stop studying, which is generally when your final grade etc is entered in the unis system, you have to cancel the extension, as the reason for the extension no longer exists.
You risk being in overstay, regardless of what date in the future the stamp in your passport say you can stay.
Talk to the school ASAP and figure out what to do. I am myself doing a bachelor's in Bangkok, and this exact issue has burned several students when approaching immigration later.