Contrary to an urban myth, Albert Einstein was actually very good at school, right from the start. He joined school aged six, straight into second grade. He was considered well-tempered, reserved, smart and driven. He was also described as a dreamer, keeping a sceptic's distance to people and material things.
Einstein's marks in secondary school in Munich were never below 2 (where 1 is best and 6 worst), including Latin and Greek, although he was said to be quiet but content. He left secondary school without an Abitur (i.e. without graduating) to follow his parents to Milan.
Source: Fölsing, A. 1995. Albert Einstein. Suhrkamp Verlag. p. 29-41.
His next formal learning experience is in Switzerland, where the marking scheme runs from 1 to 6, where 6 is now best. Again, he scores top marks in Geometry, Algebra and Physics, with no mark below 4 except French, where he scores only 3, having almost caught up three years of French in one.
Source: 1987. The Collected Papers of Albert Einstein, Vol. 1. Princeton: Princeton University Press.
A short summary of this urban myth (in German) can be found here.