Depends on the objective.
First of all, almost anything will work if you progress a little each week along one dimension (e.g. either cut recoveries, or run faster, or add/lengthen reps).
I would look up the Daniels tables, or if you don't have it then use this:
http://www.runbayou.com/jackd.htm
Now, decide if a session is 'tempo', or aerobic capacity intervals. If tempo, go at roughly the pace above. You can either have them do a nonstop run, or do reps with SHORT rests. Thus, 30sec to 1min, or maybe 100m jog on a track. At this intensity they should not be getting into oxygen debt, so they don't need a long rest - but it's often mentally easier to do a few mile or km reps with micro-breaks than a nonstop run.
If it's intervals for general aerobic development, the easiest approach is to train at 5k pace or a bit slower. Very roughly speaking, the first minute of a rep may be wasted as the heart isn't up to speed. So, focus on building the length and number of reps over time, with a comfortable rest (say, 3 min).
If you want them to race hard, then you let the reps start to get anerobic and tough. This could be done either by speeding up or by cutting the rests. Be clear in your mind and with the athletes which way you will progress it.
In college we used to do 6x3min regularly. At the start of year we'd typically have 2min rest and run controlled. As races neared we'd be on 1min rest, running hard and finding each rep harder as recovery was incomplete. Some of that is needed to get ready for 5k or XC races.
Hope this helps.