I investigated a similar idea for a conworld once and ended up rejecting it.

AFAICT ziplines with modern technology really aren't good at covering long distances. I didn't study the math, but just eyeballing existing long-distance ziplines it seems you need approximately 1 meter height for 10-20 meters traveled. The average distance to school in the US seems to be between 1 mile and 5 miles depending on who you ask. Let's take the lower option. To go 1 mile you'd need a 250-500 foot mast. But that's just on average; some people will live two miles away and need 500-1000 foot masts - up to as high as the Stratosphere Tower in Vegas.

Not only do you have to pay for a Stratosphere Tower on every block (there are 72000 blocks in Manhattan!), not only do you have to tolerate a forest of huge towers that will probably lower land value, but you've also got to get kids up a 500 foot tower every morning, which means realistically that you're paying for some really good elevators. And we're still only saving kids a two mile bike ride or five minutes waiting for a bus!

You could limit it to the blocks closest to the school to decrease max tower height, but that would also limit the benefit.