When I was a newspaper photographer, I was often called upon to go take pictures of children at the local schools. At first, I had trouble. I couldn't get the kids to behave. I couldn't get them lined up correctly. I couldn't get them to smile at the right time. But eventually, I learned the necessary tricks-of-the-trade, and because I'm such an insanely nice guy, I'll pass those tricks on to you.
1: Be stern. This is not the time to be their buddy. They may clown around and misbehave, and you don't want chaos ruining your chance of getting the shot. Bark orders and make it clear that you expect to be obeyed. If you've got a class clown who wants to disrupt things, favor him with The Glare, lower your voice an octave, call him "Buttercup", and ask if he really wants to be in this picture. More than likely, he'll settle. If he doesn't, remove him from the group and don't take his picture. You must be in control, or you'll never get the picture. However, while keeping control is important, it is also important that you...
2: Don't be a bear. There is a big difference between being stern and being mean. If you act like a monster, none of the kids will want to have their picture taken, and your photos may feature a bunch of kids who look frightened and unhappy. If you have a nice, cooperative group of children, feel free to toss off a joke or funny comment. They will be more at ease and more eager to work with you to make a good picture.
3. Get their attention. This is not the time to be a shrinking violet. Speak up and make sure the kids can hear you and are paying attention to what you say. Don't be afraid to make gestures and talk with your hands. You need them to listen to you, no matter what else is going on around you -- clowns, puppies, carnival rides, whatever. If their attention starts to wander, increase the volume of your voice and the extravagance of your hand gestures. Try to be more interesting than whatever's distracting them, or, if you can't do that, remind them that they can't play with the puppy or ride on the merry-go-round 'til you get the picture.
4. Line them up right. No matter how large or small your group is, put the tall kids in the back, the short kids in the front, and the tallest kids on each row in the center. Never let any portion of a child's head be covered up by the kid in front of him -- make sure each child is clearly visible through your viewfinder. In fact, it's best if each child is standing between the shoulders of the two children in front of him. If you have access to risers or steps, use them; if not, have some kids kneeling, some seated, some standing, and some standing on chairs. Always make sure they are holding their arms at their sides; you don't want them looking like they're holding their crotches.
5. Get 'em to smile. And don't tell them to "Say Cheese." Seriously, that doesn't work as well as you'd think. Aside from the fact that older children refuse to say it, you may snap the shutter while some of them are still on the "ch" sound, so you'll get a picture with a bunch of kids either puckering their lips out or baring their teeth at the camera. What you want is genuine smiles. For younger kids, tell them to say "Guacamole," "Spaghetti," "Macaroni," "Spumoni," or something similar. They still end in the traditional "ee" sound, and they're funny words, so the kids will be smiling anyway. Unfortunately, older kids won't say "Guacamole" either; with them, you'll probably just have to count backwards from three and hope they'll smile purty (For football players who think they'll look weak if they smile for a photo, tell 'em they all look real butch right before you snap the shutter -- at least a few of them won't be able to keep from grinning. Don't try that with the county commissioners, though; they won't smile for anything).
And one more... Sometimes, parents or teachers will try to assist you. Usually, this will be very helpful to you -- a principal's Glare is much more intimidating to a class clown than a photographer's Glare, and an observer can often help you line kids up faster than you could alone. But sometimes, they can be more of a hindrance than a help. Usually, you should ignore them -- they rarely do any real damage, and it's more trouble to shoo them off than to let them pretend to help. If they're really being a bother, tell them quietly, politely, and diplomatically that you'd rather work alone -- practice beforehand so you don't sound like a boor. Unfortunately, on very rare occasions, it may be necessary to drop all illusions of manners...
I once had to take a series of photographs of a large group of children while dealing with the very-unwanted assistance of a parent who stood right behind me and made faces to get the kids to smile. Quite useless, since "Guacamole" was working like a charm. Then she upped the ante to making "rabbit-ears" behind my head. Unwelcome and unprofessional, but I didn't do more than glare (lower-case) at her once or twice. Finally, just as I was preparing to snap the shutter for a picture, she said, "Everyone laugh at the fat man!" I immediately lowered the camera, turned toward her, clicked on The Glare, clicked on the Voice of Thunder, and whispered, "What did you say?" The kids got dead silent. The teachers got dead silent. The cafeteria workers in the back who shouldn't have been able to hear me got dead silent. The color drained from this lady's face, and the principal quickly dragged her out of my way. When I turned back to take the pictures, the kids didn't give me a bit of trouble for the rest of the day. All in all, one of the more satisfying photography sessions I've had...