Hi, Natalie. I just reviewed the Lesson Manuals for Levels A and B. In doing so, I only found 2 lessons in Level A that has to do with inches and only 5 lessons in Level B that has to do with inches. All the lessons looked easy to convert as the student is basically learning to measure and calculate a perimeter.
The program actually learns both. Sometimes it uses centimeters (with centimeter cubes, for instance) and sometimes inches (with the inch tiles). We're in Canada, and while we're officially metric, it's certainly useful to learn both -- inches and feet are sometimes more practical than cm and meters, and being so close to the US who are stubbornly non-metric, it's good to know how to convert.
If you prefer to not do inches at all, it's very easy to just do any measuring activities in centimeters whether or not that's how the instructions go -- you'd just have to mentally change the answers as needed. (And since we're only talking about elementary math stuff here, that should't be too tricky). But - as with many math curricula - part of the measuring lessons involves measuring with 'non-standard' units. This line is 5 paper clips long, this desk is 20 pencils long, etc etc. What matters is the concept of 'how many units' rather than 'how big is the unit' at first. Then you learn that some units are more useful for certain tasks (cm for small lenghts, meters for big ones, etc) and that there are some standard units so that everyone can communicate efficiently with each other. So it would not be inappropriate IMO to, at that point, do some measuring exercises with inch units as an example of a 'non-standard' unit. And maybe also measuring it with the standard unit of cm (since the early measuring is all done with physical objects) as well as with paper clips etc, and then talking about which is more useful, etc etc.
Anyway, long story short -- don't let a few inches turn you off this program. ;) It's easy enough to compensate for it.