Contrary to popular belief, Einstein wasn't a bad student at all. Apparently, that's something that real bad students made up, because he got excellent grades. His certificate of qualification for university matriculation—what in Europe is called A-levels—demonstrates this.
The fact is that Einstein was a brilliant student since he was a little kid. He had a problem with authoritarian teachers all his life, however. One example: when he was only five, he threw a chair to his tutor because he couldn't stand his style.
Einstein started going to school at age six, in 1885, where he got brilliant report cards. His mom told this to her sister in a letter:
Yesterday Albert received his grades, he was again number one, and his report card was brilliant.
But when he changed school he ran again into problems. Again, he couldn't stand the authoritarian style of some of his teachers and his attitude turned them against the young genius. That's where his bad reputation as a student comes from, after one of his teachers said to him that "never will he get anywhere."
Understandably, when he tried to get into the Swiss Polytechnic in Zurich, he failed in areas like history or geography, so he went to a new school in Switzerland in October 1895 to learn more about these.
The next time he took his A-levels, his grades were excellent (6) for the most part. Only on French he got a poor (3) but then again, who cares.
And no, nobody remembers the name of the professor who said he will never get anywhere. [Einstein Website—Thanks Edu!]