If you have Adobe Reader XI (or Acrobat) you can use the "Draw free form" (pencil) tool under "Drawing Markups" Comment tools. Go to menu View -> Comment -> Drawing Markups and choose "Draw free form" (pencil) tool to do this.
After selecting the "Draw free form" you can press F8 to hide Toolbars and F9 (Windows) to hide Menu Bar. Press F8 and F9 to show them again.
If you right click the drawing markup you can change its properties, such as thickness, and make them default.
When you lecture off you can place a drawing markup to highlight whatever you want and Ctrl-Z do remove it.
