Three points:

1 - Do not paint all teachers with a broad brush. There is a small and growing number of USA teachers in K-12 that are using 3D printers in their classrooms. Further, there are many teachers at secondary institutions worldwide who are using 3D printers in the classroom and for research.

1a - Never paint any group of people with a broad brush.

2 - A major inhibitor is the lack of "programming," the tools that surround the printer that must not only make it work but also align the printing process and/or output with the curriculum. If a teacher must create grade and subject specific content then that slows down the adoption process.

3 - Teachers and school systems cannot simply buy a 3D printer (assuming they even have funding) but they must also find time in the day for a creative and time-intensive process. I daresay that most USA teachers regardless of grade level, at least in K-12, will tell you that their day is filled to the brim with required, non-3D print related content that must be taught in order for students to perform well on the numerous standardized tests that they must take.