Not sure I can give too much advice, but try getting them into the classic sort of 2 or 3 deep line-up, straight on to the camera to minimize depth of field problems, and as far away from any distracting background, and all standing fairly close together. Fill-in flash will be useful if you have a flash - expose as normal and under-expose the flash by say 2/3rds of a stop to lighten shadows.
Otherwise, incapete speaks the truth.
Take plenty of shots as getting them all to smile, or have their eyes open at the right time is not easy. If there is a helper on hand, try to get them to 'control' the lads while you concentrate on the technical side - I try to look out from behind the camera just after taking a few and then take another shot - this often results in faces and smiles relaxing as they think its all over. Try to have a couple of jokes, and try to take charge - people will do as told if you seem to be in charge !
good luck !