If you have a DTV on soft power then just show your updated 6 months schedule and classes that's what it's designed, if you are actually using the DTV correctly and enrolled in a class nothing to worry about, if you are trying to wing it and blag then you will get caught out and possibly the DTV cancelled
Are you planning to stay in Thailand longer then 90 days ? If not you can enter on visa exempt 60 days and get a further 30 days extension at immigration, DTV isn't a holiday visa and requires conditions, see embassy website for information