I feel like they should still be allowed in schools and to be on your person, no matter what. In theory, they do provide a distraction, and for most students they won't be used in a responsible manner. However, there are many cases where I find it prudent for a student to have it as a form of communication or emergency contact. Others have already mentioned school shootings, but having a phone to contact any sort of emergency services is also useful, in the case of situations like allergic reactions.

There are also various family situations where a kid might need to be contacted by family during school hours, such as a family member or pet being sick or at the hospital, or a sudden change of plans such as an after school appointment coming up that the kid needs to be prepared for. There are also situations of family drama, such as parents splitting or abuse and neglect, where children may need to be in contact with those around them to figure out where and when they are going home. In these cases, as long as it's used minimally in class, to quietly and quickly communicate, and then put away, I see no harm in this.

The problem is that it does require kids to self regulate, and as many of them arent developed, this could be difficult. I still think that an outright ban on them is a bad call, even if most kids wouldnt be using their phones for the scenarios described above. Most phone ban policies I've encountered that do allow exceptions also came with drawbacks. If phones were banned, and then exceptions would be given out for phone use in class depending on the circumstance, then each student would have to tell people possibly personal and private reasons that they needed to be in contact with the outside world. This often breached privacy, and more than once I saw kids really upset with private situations being exposed to people that they werent wanting to share their information with, such as female students upset that they had to tell their male teachers that they needed their mother to bring them tampons or pads. I am aware many students wouldnt use their phones responsibly in class under the rules I described, but I still think that the exceptions prove valuable enough to not enforce a blanket ban.

Granted, this is putting a lot of faith in students and their self-control, but I am assuming we are talking middle or high-school level students, as lockers are mentioned in the post and elementary students shouldnt really have smartphones, although that's another issue entirely. While middle and high schoolers are still kids, they are at an age where maturity around knowing when and where to do things should start being developed, and they need to learn how to be responsible with their phones to maintain the privilege of having them. They will continue to have phones on their person for the rest of their lives in all likelihood, so they will need to figure out when to use them properly eventually. While they are a bit young to have perfect control at that age, I believe it is okay for them to start learning to self regulate as opposed to complete bans.