If you make them ignore everything happening outside school between students that have to work together at school, nothing good will come of it, either.
There’s a difference between a victim receiving a threatening text and showing it to administration, and the school demanding to see random teenager’s phones to look for potential crimes. If they have a suspicion, get an actual investigative police detective involved.
Do you think teachers trained to investigate and properly handle evidence? Because if they don’t, then the evidence isn’t admissible in court, or maybe they decide to kick out the wrong student on suspicions.
Every day, administrators already have to investigate occurrences at school and choose appropriate disciplinary paths forward.
Coursework in legal matters is part of educational leadership/administration programs and is a focus of professional development, in part because districts are incentivized to reduce litigation costs.
I share your apparent concern that districts, etc, screw up discipline often and unjust outcomes occur. However, doing nothing isn't an option, and I don't think punting more of school discipline to the police is likely to improve outcomes.
If parents agree then the school can have access, however the default should not be that they can demand access or punish parents for not providing it.
IMO, if you are bringing a phone to school, and there is a substantiated allegation of a violation of a reasonable school policy involving the phone-- examining the phone should be fair game. And, this is what appellate courts have held.