I'm not a parent, but I don't think it's appropriate for kids to be running around with replica weapons in school. Maybe a little hypocritical, but times have changed. When I was a kid we used to make our own bow and arrows, and we'd run around shooting eachother with them. Nowdays, you have Columbine, Taber, and Virgina Tech to list a few. Kid shows up with a replica whatever (grenade, pistol, etc), someone overreacts or misinterprets the situation, and someone gets hurt or worse. Seems like every so often a kid get shot over a replica gun by a cop who can't tell the difference. Why tempt fate?
And to argue along the same lines about calling ahead to let the teacher know, that's just one person out of many that may come into contact with the replica as the kid goes into school. Does the teacher mention this to the rest of the school faculty? What about the bus driver? Or kids in other classes? Some citizen who doesn't have their head up their behind on public transportation and notices this? Sheerin is right, the only people who should be "educating" others about the hazards of live or dummy rounds, or grenades and such, are trained professionals.