It has been few months they changed the requirement,  they do not need any letter from the school. If you have minimum of six month date on I20, you are good to go. My community college I20 is expiring in december and I renewed my DL a week ago. But they renwed it only till december, with the exact date on my I-20, which was fair enough. I went to Irving but I suggest go to any other DPS, you will be done in less than 30 mins.