I’m not einstein in any way shape or form but I have a bachelors degree in math and I got a D in Calculus when I was in High School. I would still flunk out of high school calculus at this point. Anybody independantly looking at my transcript would think I was bad at high school calculus, but truth of the matter is that I simply did not do the homework I did not feel was necessary to complete my understanding of the matter. That left a rather idiotic looking pattern on my high school transcripts of actually me having the worst grades in classes I was best at.
So, analysis of Einstein’s mathematical abilities as a child should probably avoid things like grades or degrees and look for a subjective written opinion of his analytical abilities that actually discusses. I’d guess such a thing is probably very hard or impossible to find at this point. Currently grades are typically very heavily influenced by homework, and homework is typically graded based on assigned work and not just attempted work. As a person who has several times eked out a B- in a class with homework being worth 20% of the grade by doing a fifth of the assigned homework and getting 95% on tests, I always feel compelled to argue that grades are never and can never be a sole indicator of aptitude in any field.
So have I. See my contribution (Post #5) in this previous thread: Did Einstein really flunk? (And you can read the rest of that thread, while you’re at it.)