I have everyone do a mile time trial. (But, I really like what someone wrote about a two mile time trial as a more accurate gauge of fitness.)
For newbies, I have them run Daniels' Cruise Intervals at Marathon Pace. Also, I have them run by time & not distance. So, it'd look somthing like this: 3 X 7:00 @ MP (marathon pace) w/ 1:00 rest. This works for two reasons: 1.) it allows them to learn pace & reset after each interval and 2.) it isn't so stressful on their under-developed aerobic system. (After 3+ sessions at MP, you can possibly move them up to a true Cruise Interval at Tempo Pace.)
For veterans, it looks a lot like the above only I use Daniels' Cruise Intervals at Tempo Pace. So, it'd look like this: 3 X 7:00 @ TP (tempo pace) w/ 1:00 rest.
As a coach, by going with TIME & not distance for Cruise Intervals, everyone starts & ends at the same time. (This helps keep everyone together & your practice organized.) And, everyone gets-in a 21 minutes tempo run.
I should also say that I only have about 25-30 athletes & I give everyone their 400, 800, 1200 splits for their workout. (My athletes either memorize those numbers or write them on their arm.) They have running partners too (for pacing help). So, I stand at the start/finish line & simply go through the list--set up in fast to slow order--and tell them as they pass me if they're running too fast or two slow.
Good luck!