What I would do is go the simplest route. Single flash with umbrella centred just above my head and pointing down towards the student. Have the flash/camera at least 10 feet away. Mark line or X on the floor where people stand. If they sit on a stool that is OK too.
Camera on manual, F8, 1/160 and ISO 400. You could even put white cardboard/paper on the floor between yourself and the student so some flash bounces off the floor and reflects up to light under the chin. Only really effective with a shoot through umbrella which I use for informal portraits. I use a reflector and raise it a few feet off the floor.
As for flash put in on manual like he's gone said. If you can get the whites to expose properly the rest of the exposure falls into place. Skin colour does not matter. Use the white towel method. Scroll down and look at the gent holding the white towel. Adjust flash power so the white towel part of the histogram looks the same as the image. Don't let it hit the right wall, just get it as close as you can. Once you establish this then just shoot away. No need to do it every time as long as the students are same distance to you. Thus the X on the floor.
If you use a white backdrop be careful the histogram is not giving you a false reading from the background. An light brown or blue might be better but white will work. Is there lightly coloured wall you can use?
600 students? Like he's gone said have plenty of spare batteries for the flash and recheck using the white towel method about every 50 to 100 students. If the flash plugs in an electrical circuit then less checks are required. In fact you should be able to go all day after the initial set up.