Maximum ride time governs everything
In employee transport, ride time is a comfort measure; in school transport it is a hard limit. For younger children, time on board is expected to stay under a certain threshold, and when it is exceeded the problem comes back not as a planning error but as a parent complaint.
That limit directly caps how many students a vehicle can carry. A vehicle with capacity for 16 may have to run with 11 because of the ride-time limit. A plan that tries to fill capacity hits that wall here.
Merging stops is usually off the table
In employee transport the walking distance is a dial you can turn; in school transport that dial is usually locked. The student is expected to be collected at the door, so the number of stops is close to the number of students.
This makes the route extremely sensitive to the order of stops. Swapping two addresses costs a few minutes in employee transport; in school transport it can be enough to breach the limit.
The calculation runs backwards from the school start time
The only fixed point in school transport is when school starts. The plan is built backwards from there: arrival time, then the last student's pickup, then the one before, until you know when the first student must be collected.
That first pickup time is what most planning arguments are actually about. A route that starts too early upsets parents; one that starts too late arrives after the bell. What settles the difference is the number of vehicles — and the number of vehicles is the cost.
Sharing vehicles across schools
Students living in the same area but attending different schools can share a vehicle if the start times allow. This is the one structural change that markedly reduces the number of vehicles on the road.
Whether it works depends on how close the start times are and how the schools are laid out geographically. Hand-built plans rarely attempt it, because weighing two schools' lists together is far harder than planning one.
School transport routing is the planning problem where constraints push against each other hardest. Filling capacity, shortening ride time and reducing vehicle count cannot all be demanded at once; a plan built without deciding which comes first loses all three in the field.