What are the implications of submitting a paper to be graded and to arxiv preprint on the same day? I submitted my paper to be graded in a university class and to the arvix preprint on the same day. Are there any implications for the grading professor and myself if the arxiv preprint is announced before I am graded?
No, there are no implications. Professors grading your work don’t care what you submit to arXiv, and the arXiv doesn’t care what you submit for grading. At any university I have ever had experience with, those two things would not affect each other in any way.

But what you did still doesn’t make sense to me. The only type of work that’s suitable for submitting to arXiv is a research paper. And I have never heard of a student submitting a research paper as a class project to be graded - it’s not that that’s not allowed or anything but research papers usually materialize as a result of extended effort that goes outside the scope of work done for a specific class. It would be a bit like submitting a class project that’s, say, five times longer and harder to write than the requirement. You would get an A, surely, but again, for obvious reasons that’s not something you ever see students doing.

(Edit: according to the comments, writing research papers for a class project is not uncommon in some disciplines, or at least may have not been uncommon several decades ago.)

My suspicion is you have some misconception about the benefits of submitting your class paper to arXiv. Perhaps you should rethink whether that’s something that you want to do and/or consult some experienced people about whether it makes sense. But regardless, for your actual question I don’t see any problems for you related specifically to the simultaneous submission to arXiv and to the professor.