Around 50mm Focal Length on a "Full Frame Camera" is good for a group of 12 to 40. Pack them tight but comfortably and the PERSPECTIVE attained when filling the frame using a 50mm lens is visually attractive and also provides a good a SUBJECT DISTANCE for Direction and also a reasonable chance of being withing the Maximum Working Distance to use a ONE SPEEDLITE Flash, as the Fill Light (outside); and Flash as Key Light (inside)
I use a Tripod for Group Portraiture - not for stability of the camera: but so I can frame the shot and then stand away from the viewfinder being able to see and then direct all the heads and eyes toward the barrel of the lens.
If you do not have Flash and are not versed in its use then Open Shade (as per Manfred's example) will probably be the easiest unless the Classroom has reasonably bright DIFFUSED SUNlight. Note Manfred's Shutter Speed - don't go slower than 1/250s: 1/320s is safer; 1/400s is safer still for
Available Light Group Portraits.
If you do choose to use Flash then you really need to understand what you are doing. Shooting inside, the Flash will usually be the Key Light, but shooting Outside the Flash will usually be the Fill Light. Both scenarios present (possible) challenges to the novice: inside you need to have enough even spread of Flash to avoid hard shadows and on camera Flash and Ceiling or Large White Card BOUNCE MODIFIER using ONE Speedlite is the simplest technique. Using Multiple Flash Heads and DIFFUSION MODIFIERS is more complex but if done correctly can make excellent results. More complex again is to (sometimes) gel the Flash (or the windows) and shoot with a mixture of window Light and Flash making the exposure, (sometimes dragging the shutter) - I would not suggest this technique for a school group this technique is more for the "artistic" portraiture to capture the room 'ambience'.
Outside in Hard Sun or Open Shade, Direct Flash is is most common technique. The main issue when shooting in Hard Sun is the lack of necessary the power to 'Fill" using a single Speedlite Flash - As a Rule of Thumb you need the (most powerful) 'Speeldite' Flash to be situated at about 4mtrs (12ft) or closer to the group for the Flash to have even a minor effect as Fill Light when shooting in Hard Sun.
If you do shoot outside then early to mid-morning or mid to late-afternoon will be the more creative time apropos The Light - but for Portraiture of School Children (and any group really) - early to mid-morning is the very best time, because you have the best chance of the maximum number of Subjects being neat, tidy, happy and focused on the job at hand.
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Here are some illustrations of various techniques:
The above is shot early-mid morning with sun behind the Subjects and on Camera Flash as Fill - at this time of day and with the assistance of a light cloud cover, it is well before the sun is at it strongest - that means the AMBIENT exposure is around EV = 13 and that means the Flash does not need to work as hard for the Front Fill - 50mm on Canon 5D Series camera.
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Moving inside and a very high ceiling meant it was impossible to using the Ceiling for BOUNCE - and in this situation a LARGE WHITE BOUNCE CARD was used for the Key Light and to balance the bright sunlight coming through the windows at the rear. A 24mm Lens on a Canon 5D Series was used - a wide lens was necessary to allow the (on camera) Flash close enough to have enough power to balance the hard sunlight streaming through the windows at the rear and also a wide lens was required to get the camera high enough to avoid any noticeable
Flash Fall Off through the group, front to back.
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Inside with a ceiling low enough for on camera single Speedlite CEILING BOUNCE. Note the area of Flash illumination is a different Colour Temperature to the background - this is good for a small group and usually for Group Portraits which are in a "less formal setting" - Probably this would not be a good idea for a classroom - so if you shoot with Flash in the classroom, then consider turning off the ceiling lights especially if they are FLUORESCENT or other domestic lights with a strong/obnoxious COLOUR CAST. Just allow DIFFUSED window SUNlight to be the Ambient Light in the room and balance the AMBIENT EXPOSURE to be about ˝Stop to 1Stop under the FLASH EXPOSURE and that will give a nice "POP" to the Group as they contrast against the slightly darker background - per the example above.
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This is inside using only the ambient light - notice the lighting is quite flat - which can be good for a big group because there will be no hard shadows imposing on faces of their neighboring Subjects -
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And this is an example of the lighting that you do NOT want for a Group Portrait – whilst the definition, depth and modelling created from the hard sunlight can be used to enhance the dynamics of a Portrait of Single Subject – we run into all sorts of trouble with hard shadows falling on faces when making a Group Portrait – so – returning to the beginning and as per Manfred’s example use a nice area in Open Shade.
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