Do I need 35 hours of ground school for knowledge test for PPL?
For 141 you need a graduation certificate to take the test. For part 61 you need an instructor endorsement. 141 is for complete courses start to end. 61 is for getting a piece here and a piece there mix and match style. in either case there is a minimum amount of ground training required before you can get your private certificate.

Any either case you don't need the knowledge test to start flying, you only need the knowledge test before your final FAA check ride.