I was asked to take individual portraits of around 15 elementary age students who missed picture day in school . I will be taking with a Canon 60D camera & Canon 430 EX II flash. The room will be lit by fluorescent lighting. I will not have the luxury of using a tripod. what recommendations can you suggest as far as which lens should I use (Canon 24-105 MM L lens or Canon 50 MM lens)? . Preferred aperture/ shutter speed? Direct flash or bounce?
You have several things to think about.
First off, you would probably like the photos to closely match what the photos of the other students look like, so pay attention to the background, the pose, and the lighting angle in the other photos.
Secondly, you have to "take out" the effects of the fluorescent lighting if you do use the room you mentioned. Fluorescent light is very hard to white balance, it's ugly and it also flickers potentially creating color casts as mentioned above. If you can, turn off those lights. If not you can use the shutter speed to reduce the impact. (See second last point below.)
Thirdly, you don't need two flashes. You can use the 430 EX II that you have, but you'll want to use your 60D with the 430 EX II in wireless flash mode. (Refer to page 139 of the Canon 60D of the Instruction Manual.) I would mount the flash on a stand and shoot into an umbrella, placing it 45 degrees to the left (or right) and above the subject. Then I would place a white reflector (it could be a large sheet of white paper or bristol board or foam core) on the opposite side of the flash next to the subject.
Next, I would bring both lenses. If you can use the 50mm, great, however, without knowing the dimensions of the room, the flexibility of the 24-105mm may be required.
As for shutter speed and aperture, I would put the camera in manual ('M') mode and use a shutter speed of 1/250 to take out the effects of the fluorescent lights if you can't shut them off. If you can turn them off you can use shutter speeds of 1/250 or slower to have some control the lightness/ darkness of the background. I would start with an aperture of f/4-5.6, but the choice depends on the background (plain vs busy). The E-TTL between the camera and flash will take care of the flash output for you.
Finally, test, test, test with a volunteer subject, before you do the shoot.
First things first, use bounce flash if you're doing head shots. Then use the 50mm. With the crop factor of the 60D it will give an 85mm equivalent.
If you can, take the shoot outdoors; that way you can use natural light. Or, place your subject near a window and use your flash as a fill light.
You may not like this, but give up.
With only one bouncing flash, you will end up with a very flat picture. Moreover if the bouncing surface has some color, it will tint the light.
With one direct flash, you will get something way too dramatic for the purpose of these headshots,
With different lightsources or a flash as a fill-in, you will get different colors casts on different parts of the face (different white balances) which is a mess to recover in post production.
With only natural light, well, toss a coin because you never know what you will get (harsh sunny day or diffused overcast).
With only fluorescent lighting, your pictures will look sad and washed because the color spectrum of fluo bulbs is not full so the color rendition will be off (especially with the cheap bulbs used in schools).
You need at least 2 flashes and softboxes or umbrellas or one flash and one reflector to do a decent job. Of course, I'm talking about off-camera flashs.