TEACHING LOADS for Librarians could we decline?

I would like to seek who can interpret the Magna Carta for Teachers.

I am currently a school librarian in a private school. My principal gave me two subjects (grammar & creative writing for high school) to teach that is not in line with my work. I sent a letter to inform her that I decline to continue teaching the subjects so I could focus more on the task at hand which is being the librarian. She responded that under RA 4670, a teacher status applies to a school librarian and I am required to teach not more than 6 hours a day. I have an overload pay for those two subjects but I find it too low for the responsibilities that go with the teaching assignment.

Now she would like me to justify why I should not accept the loads given to me. Could somebody enlighten me on this matter. Thanks

RA 9246, on scope of Librarian practice,says
xxx
5) Teaching, lecturing and reviewing of library, archives and information science subjects, including subjects given in the licensure examination

i dont think that will fall under archives and information science subject, save if they are one of those subjects given during licensure exams

could i use that (RA9246 -Scope of librarian) to justify my decision?