These are mulberries (Morus Alba, or Morus Rubra, I can't figure out how to tell them apart). Both types are often used in the making of jam and jelly, pies, or eaten fresh. They are usually considered non-toxic, though eating lots of the completely unripe (greenish-white to pale pink) fruit can make you sick, as is often the case with consuming under-ripe fruit (witness the phrase 'green apple quick step', used long ago by my scout master to describe... well, you can figure it out). In this case, however, consuming the completely unripe fruit or the sap of the tree can lead to intoxication and hallucinations. So, if you've got a bunch of 3rd, 4th and 5th graders picking the berries, it might not be an over-reaction to discuss the situation with poison control. Just thought you'd want to know.