Here are two examples of morphing the old paradigm of "library time" into innovative instruction in the library. These two simple examples model adding rigor to transform an old passive "storytime" into a time for close reading, examination of the text, and discovery of meaning.
If you really want to shock your building principal, consider asking him for the "item analysis" from annual testing, by grade level. Tell him that you'd like to contribute to the weak areas and target a few instructional periods to address items such as inference, decoding vocabulary words, main idea and supporting details, and more. After suggesting this once locally, I had a new librarian share that her principal looked at her in shock and said, "I've never had a librarian ask me that before." (Instant SCORE!)
Another way to foster the "instructional partner" paradigm is to eliminate the "book exchange" from your scheduled library time. When I was an elementary librarian (back in the Neolithic Age), all books were "returned" to the library in the mornings. That way we reduced our library book exchange time by 5 minutes per class. That may not sound like a great deal of time, but every 5 minutes counts. I've heard of schools where kids who want to check out a book, have to come during lunch, recess, before or after school, or with a library pass. Every situation is different, so only you will know what works for you.
The research says that the "more kids read the better they read. The better they read, the more they'll comprehend. The more they comprehend, the higher the achievement."* Get those kids reading. Increase your circulation. Enlist the help of honor society students needing volunteer time. Let them shelve your books! Increasing your instructional value is the best way to advocate for your position!
*(Self-quote from "The Importance of Increasing the Volume of Reading" written for Expeditionary Learning, NYSED https://www.engageny.org/resource/the-importance-of-increasing-students-volume-of-reading . 'Based on the research of Marilyn Jager Adams)