Just a few thoughts that I have used for similar shoots, but they may help you get started....
As said above, look for useful backgrounds though from my experience, schools often have very busy walls so you may want something less hectic.
If natural light isn't possible then find a white wall and use that to bounce your flash from. Stand 3-5ft away from the white wall, aim your flash at it and have your subjects standing a similar distance away from the wall. Put your grey background a few feet behind them so it fills the frame but doesn't get too much light on it, or use whatever background you have available. Remember that the light from your flash will fall off fast, so any background ought to be fairly underexposed so darkish - look for windows or other sources of light and close off if needed.
A stool could be of use for keeping the pictures all fairly similar as the lighting will be much the same for all the pictures, and head and shoulder shots will be perfect given what they are going to be put to. Try whatever poses you feel will suit but facing them slightly towards the white wall but with faces towards you is a good starting pose. A tripod and remote release may help as it leaves you free to chat, and people are less able to see when you are about to take a shot so it is easier to get a relaxed expression. I use a battery pack to give faster recharging but if you up the ISO a little that will help and allow you to shoot a second shot just after the first which may also help. A reflector (tin foil on card, white card etc) taped to a small stack of chairs or some sort of stand will bounce a little light back into the shadows, or if slightly behind the subject may help separate them from the background.
If possible, try getting it all set up in advance so that you can see exactly what you are getting then you can relax in the knowledge that the technical side is sorted and concentrate on the subjects. Work fast, ask questions and explain what and why you are doing this or that - again, just my way of working but it seems to work some of the time...
Good luck !