The RER ( the metro is the system that only operates in zones1-2

) is the best way to go
The RER A from MLV-Chessy (
Disneyland Paris) will take you 40 mins to go to Chatelet where you change platform to the RER B and go one stop to St Michel.
From there it's a very short walk to rue de harpe ...you can check it out on google street view the walk from the station
The section of RER A coming from DLP is always busy so you'll not be on your own in the carriages.
I'd advise you to avoid rush hour if you've got bags but other than that you'll always get a seat
Seeing as the parks are closing at 22.30/23.00 you'll have lots of people going back into Paris even then.
The only "nuisance" you might get would be the musiciens or a guy coming around begging. Just ignore them, they are harmless and non insistent
