The best model I can think of is that of movie ratings although much more detailed. Use of this model would be familiar to parents.

I think is it burdensome to set up a system requiring parents to read any book before the child can read it. Once the books are rated, the parent can be notified and respond. Just providing the book to the parent to read is burdensome both to the parents and to library staff. 

First, all books should be rated. Examples of ratings for Book A might be: "Contains Description of Violent Rape or Sexual Assault; Contains Description of Unprotected Sex. Recommended for Students aged ... and above.  Book B might be rated: Contains description of Anal Sex; Contains description of use of Sex Toys. Recommended, etc.

Having a rating does not mean that parents cannot at any time review the entire library of material containing explicit sexual material and they can request a blanket rule that their child is not to read material with any sexual content. In addition, it should be understood that any book with ratings is not to be shared.

You could also develop similar ratings for books containing CRT, transgender propaganda, and Marxist/communist propaganda. 

Once put in place, this system would function with a clear and familiar purpose and put the least stress on parents, the pupil, and librarians.

Good luck!

Chris