Teaching is definitely a socially valuable activity, but in my experience, K-12 teachers are often particularly uncomfortable with the flexibility/uncertainty of fair use (for a variety of reasons), and quiet a few literally never copy anything for their students without permission.
One thing I am absolutely certain of about fair use, is that it definitely allows teachers to copy things for their students without permission sometimes. The law specifically says so:
"[T]he fair use of a copyrighted work [...] for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright" - 17 U.S.C. § 107
Lemme just repeat that.
I don't blame teachers for feeling a bit unsure about fair use. Many informational materials about fair use for teachers are aimed at risk-avoidance, and a few actually intentionally sow confusion. Even when well-informed, teachers do not always have reason to believe their school systems will be supportive of fair use in classrooms.
I do encourage teachers to question any assumptions they have that they always need permission: fair use definitely applies to classroom teaching, although not all copying for classroom teaching is going to be fair use. I also encourage teachers to raise these issues for discussion in their schools, districts, and professional organizations.
I also give a serious side-eye to people who train, support, or provide information for teachers about copyright, and undercut fair use and tell teachers they cannot or should not ever rely on it. If you're doing that on purpose, you should be ashamed. The fair use statute specifically calls out copying for classroom use from a list of other socially valuable uses. That's not a drafting error.
(Also, don't forget 17 USC § 110(1)!)