Thank you GPY. May I know why it's necessary to add on there? I understand that we're stressing continuity here but isn't the word dragging enough? Not to mention the fact that the continuous tense is used.
I would say that "dragging" by itself (in the relevant sense) is used more when time seems to be going slowly. For example, if you look at your watch in class and it says 2:30, and then what you think is 15 minutes later it still only says 2:35, then you could say "This lesson is really dragging". "dragging on" is used more when something tediously lasts longer than you hope or expect, like your c