I agree with relache - we didn't have cell phones when I was in school, and I don't think crime is all that much higher now than it was then. I'm not sure what would be illegal about it, since all field trips require written parent or guardian permission. It all depends on who is supervising the fieldtrip and if you trust them. If I trusted the chaperone, I would let either of my teenagers go.
By the way, I've been mugged four times, twice in broad daylight in a public place, and once at gunpoint. All four muggings took place before the invention of the cell phone.