This is a pet peeve of mine! In the younger yeshivos (usually litvish), the Rabbeim know that they can’t leave the class alone! In the old fashioned yeshivos (usually chassidish) the boys are left alone by recess!
I don’t think that you will change the yeshiva. Pick your priorities. I don’t think that it’s the right thing to do, yet if there are adults in the building I don’t think that there is an issue with “Venishmartem.” A boy can run for help if there’s an emergency.
I am generally happy with the chinuch of the yeshiva that my son goes to and I swallow this issue.
If you feel strongly enough about this issue then change yeshivos. I don’t think that you will get the rebbes to lose their coffee break.