You can try the NetworkCostCalculator, in order to speed things up you may want to only apply that to objects within a 3.5 km buffer of any given school.

I have something that sort of works

NetworkCostCalculator

MeasureExtractor

ListIndexer

AttributeRangeFilter

But the result is not ideal.

For MeasureExtractor I chose Type "Whole Line" which gives me a measure for the whole line, but I suspect I may be better off with Type "Individual Vertex" which I think will give me the driven distance for each vertex of each line.

I was really hoping for a transformer where I specify a driven distance and the output is the road network inside that driven distance

Yes, individual vertex would give you a bit more detail. I did something similar a few years ago for drivetime polygons (pretty much the same process), but found it a very "expensive" process. A 5-minute drivetime polygon in an urban area took about 10-15 minutes to calculate.

Have you looked at this article? https://knowledge.safe.com/articles/30048/creating-time-and-distance-isolines-using-the-netw.html

Thanks, this helps me make progress.

I am successfully creating contour lines at the interval I need.

But, I really want polygons, so that I can detect if a point is within or without the 3.5km driven distance.

How do I convert the contour lines into polygons?

More progress. I use;

AttributeFilter to only keep the contour intervals I want (keep 3.5km, discard 7km)

LineJoiner to join the contour lines end-to-end into one line

GeometryCoercer to convert the line into a polygon

Which is great!

Except that, within the polygon is a spot that should be a donut hole. But there is no hole. I have a second line outlining the hole and that is its own polygon.

How do I generate a polygon that has a proper donut hole?

Okay, so the trick to building polygons with donut holes is to use AreaBulder, not GeometeryCoercer

Specifically, AreaBuilder with;

CreateDonuts ticked (on) and

Drop Holes = Yes

this creates polygons with donut holes (but the holes are not separate polygon features)

Is there a transformer that can split a line by measure (or elevation)?

And another transformer that can filter features by measure (or elevation)?