I think dreamhippie summed it up very well. You can also use bills or any receipts that have your name and date to prove that you were here... if it comes to the point where you would REALLY need to prove that you were here during the time you were not in school. I work at a law firm and during cancelation of removal cases, this is what we submit to the court to prove that a client was in the US for "X" time period.