Unfortunately this kind of conversation held in a void is not very productive. While these releases are in general helpful and can be used, they are certainly not "bullet proof."
Furthermore, they do not insulate the school from grossly negligent, or intentional, actions by their employees (teachers or staff), and the school cannot protect itself from placing children (or parent volunteers for that matter) in dangerous situations.
The waiver is at best a "good practice" for a school as it can be a good defense against some claims for simple negligence. (If I were defending a school district and had a waiver on hand, I would certainly try to use that as a waiver of liability).
I will also tell you that most schools use those documents primarily for purposes of getting advance permission to treat. In the event that a child is injured on a field trip or outing, the school needs permission from the parent to treat the child (both with first aid, and in having the child admitted to a hospital or treated by a physician). Having these forms on hand is extremely important as it is "express consent" to treat the child.
I am not sure what the attorney friend you had was referring to with signing away Constitutional rights (defendants do so all the time in criminal court, usually against the better advice and instruction of their court appointed lawyers), but I understand his basic point - courts are very reluctant to enforce a waiver of any right when the party's waiver is uninformed, it is difficult to have an "informed waiver" when any potential injury or claim is merely theoretical (your child is going on a trip, he or she may be injured in some way, now please sign this document to waive all civil recourse against the school, which may be responsible).