If you get the following dialog box when you open a drawing in AutoCAD 2013:
Open the file in AutoCAD 2015 and save the file (2015 saves in the 2013 format). Once you open the file again with 2013, the message is gone.
Notes from Cadalyst tip review R.K. McSwain: In the past, if you saved a DWG using the educational version of AutoCAD, a plot stamp identifying the DWG as educational would be embedded in the DWG file and could not be removed. It would even "infect" commercial drawings if you mixed the two up. Starting with AutoCAD 2014 SP1 and later, once you open the drawing in the current version and save it, the plot stamp will no longer plot and/or be part of the DWG file. For more information see: http://autode.sk/1AheAsc.