I am about to start at a 141 school. Their ground school is done online via king schools. I have the app downloaded on my ipad and go through material everyday. It tracks your progress and my CFI will be able to see where I am at with all of it.
My first flight lesson is on 5/10/19. I too have heard you should try to take your written prior to training in an attempt to save time/money. I mentioned this to one of the staff at the flight school and they said they do not recommend that. He said that there are things that you are learning through ground school/books that will make much more sense once you are actually flying. I can see that being true, but I could also see them saying that in an attempt to get more $$ from students (assuming that taking your written prior to training actually does help save money/time.... maybe it does but perhaps on a case by case basis depending on how a given person learns? I really do not know...)
If your flight school is like mine in that mine uses the online ground schooling, you may be able to get a head start without dropping a lot of $$$. I paid $394 for a ground school package. This included a flight bag and the ground school from King Schools. So I have just been going through that program every night after work. I am about 25% through the material so far in about 2.5 weeks.
Something to keep in mind, I had bought the sporty's private pilot package for ground school about 1.5 months ago. Basically the same thing as the kings school, but when I visited my prospective flight school to discuss more about the flight training, I was told that they only uses kings school. So I HAVE to use the Kings School program. So I kind of wasted $300 of the Sporty's program.
If you are going to get some kind of online ground school program (Which it seems from what I have seen that a lot of part 141 do), just check with your flight school first. Otherwise you may spend money on a program that they will not use/accept.
I am still a little confused on exactly how my flight schools training goes.... whenever I have gone in person to talk to people I never seem to get clear, concise straight answers. Hopefully once I start actually flying I am able to see how the process will be for the duration of my training.