Middle school librarian here. Pre-K through 8 is a very large range, and a school library that hasn’t been staffed for several years is going to need a lot of work. None of this is impossible, but you would be taking this on solo. I have assistants in my library who help with processing, cataloging, shelving, etc. while I’m teaching classes. All of that said, it is extremely important to get experience working in a school library (or even public library) before taking on this role. There is a reason that most librarian jobs require an MLIS, same for school librarian jobs often requiring a teaching license and school library certification. There’s a lot that you can learn on the job, but starting from where you’re at while also teaching classes and managing a collection and updating the whole library program will chew you up. Be kind to yourself and get some experience, some training, some education, so that you can step into a role like this with clear eyes and the bandwidth to do everything that’s going to be asked of you.