save the file as a DXF file: go to file menu, save as, and then choose the autocad2007DXF from the file type menu (located down on the "save as" window)
once the file is saved as a DXF file, close the file, and open the DXF file, it should open without any alert, you'll see nothing has changed in the drawing, now u can resave it as DWG, and u won't get that alert.
PS: each time u copy and paste, or insert any drawing done on an educational version u'll have the watermark again, and u'll have to fix it again