When walking to school, I used to take a shortcut up the "Monte da Guia". Surprisingly, most pieces of the shortcut are still there!
Here is how you do it: Starting from the Horta e Costa Avenue, you climb up "Monte da Guia", and follow the track with all the joggers. Then, you find a little dirth path down, that takes you past some very humble dwellings with a vicious guard-dog, into a junkyard and there you are! The "Liceu" used to be right at the bottom, but there are now a couple of big boxy buildings between the junkyard and my old school.