This is a good question. I have not found any authoritative reference on whether school librarians were intended to be included as a "qualified educator" in terms of Section 62. However, to the extent that a school librarian has direct interaction with students and does instruction whether formal or ad hoc on information access and other uses of the library, it is reasonable to argue that the librarian is qualified for this deduction. The librarian's job description and actual duties may be the final determinate.
As you note, the IRS does not include "librarian" in the list of jobs that qualify for the educator expense deduction. The law is:
"For purposes of subsection (a)(2)(D), the term “eligible educator” means, with respect to any taxable year, an individual who is a kindergarten through grade 12 teacher, instructor, counselor, principal, or aide in a school for at least 900 hours during a school year." (Section 62(d)(1)(A) at https://www.law.cornell.edu/uscode/text/26/62)
The law specifically mentions "the classroom" in Section 62(D)(2):
"The deductions allowed by section 162 which consist of expenses, not in excess of $250, paid or incurred by an eligible educator...in connection with books, supplies (other than nonathletic supplies for courses of instruction in health or physical education), computer equipment (including related software and services) and other equipment, and supplementary materials used by the eligible educator in the classroom[emphasis mine]."
However, it is clear that the law lists some school employees listed do not normally do their jobs in a classroom. Indeed, the counselor and the principal normally do not work in a classroom, yet qualify for the deduction.
Furthermore, the trend in the education industry is to refer to "learning spaces" rather than "classrooms", thus including any space where learning occurs as a classroom.
In addition, even the government itself calls librarians to be teachers:
"Duties
Librarians typically do the following:
...
- Teach classes about information resources"
See "What Librarians Do" at the US Department of Labor's Bureau of Labor Statistics at https://www.bls.gov/ooh/Education-Training-and-Library/Librarians.htm#tab-2
The American Library Association describes roles of the librarian which include "teacher" and "teacher partner", wherein the librarian directly interacts with the students - see http://www.ala.org/acrl/standards/teachinglibrarians. Thus, to the extent that the school's definition of the job duties of the librarian include direct contact with students and instruction (formal or ad hoc) on information access and retrieval, then the librarian can reasonably by construed as an "instructor", which is generally a broader term than "teacher".
Thus, you could make the reasonable argument that a school librarian who has direct contact with students and who instructs students on how to use the library for a variety of purposes can be included as an "instructor" in the list of qualified educators.
