I don't see a problem with taking the phone, as long as it's returned by the end of the day. The school should have your phone number on file and can reach you in an emergency. The situations where your child would specifically need to reach you on their own phone with no other possibilities of getting help (or getting their phone back in a hurry) while at school are so uncommon as to be not really with considering.
I have four kids in school and while extreme incidents are rare, if I set my kid to school with a phone it would be to contact me in an emergency. I completely agree with no phones out, on silent, etc., but I’m unsure how I feel about schools taking away tools that could be used to get help.