After completing the technician course, am I still required to work on the technician project?

Hello community. I'm graduating this year as a building and systems technician in Freiburg. Part of this degree includes the technical work that I completed at the neighboring school. Now, I had to go into a room because that's where the system my project is located. Unfortunately, the school only granted me access at the end of February, and because of classes, I could only work there one day a week. This meant that the practical implementation was not fully completed. How could it be? Of the 150-200 hours that my classmates had, they only got 64 hours because they had access to my room (classmates completed their project work at companies). Yesterday I gave my presentation on the project, which went well so far. Afterwards, before the grading, one of the examiners (my supervisor) came up to me (he's a teacher at the school for which the project was done) and said I still had to work because the project wasn't finished. I was then forced to promise to continue working on the project after graduation so that the teacher wouldn't force it on me when grading.

In my opinion, this is an attempt at blackmail, as he's more or less implicitly threatening to lower my grade if I don't finish the work in my free time, even though it was the school's fault to grant me access to the facility! I'd been requesting access since September and received it in February… The documentation was due in March… So why should I be obligated to finish a work in my free time when it's the client's own fault that they didn't have time to complete it?! Likewise, not a single one of my classmates' practical projects was finished, but that doesn't matter to them, because the company…

Is that even allowed? It seems blackmailing and exploitative to force me to work for the school for free based on my grade. I'm open to good ideas and have good legal protection. Thanks in advance for the answers!