Twelve mics ---I only wish !!! the desk I use in the main theatre I am associated with has 56 mic inputs, plus 16 line inputs, a stack of AUX sends, is fully digital with snapshot recall and for live productions I regularly use the lot and on occasions for bigger productions I have been known to have to couple to the main desk an other desk (generally to mix the orchestra).
The initial poster I think left out some very important points (1) what type of mics are being used, (2) are the mics being hand held or positioned in the one location for the entire performance (eg on stands at the front of the stage, dropped from the ceiling/grid, etc), (3) are the mics being shared by other performers during the performance, (4) where are and what type of loud-speaker system is being used, (5) was the venue tuned before the start of rehearsals with the mics positioned on stage, (6) are fold-back speakers being used, (7) what type of technical rehearsal/s was/were undertaken, (8) how much time was allocated for technical set-up AND (9) how much time did the sound operator spend writing up a complete microphone plot before ANY rehearsal commenced.
I could go on but I think the above is enough to start with.
When I do the sound for a production -- whether it be the local dance/performing arts academy or a major performance with multi-million dollar budget --- I firstly discover the vocal strength of every cast member then I take the script (printed on one side of the paper only) and go through it line by line and determine who does or does not require a microphone and then I consider the type of microphone I consider best for that performer. After doing this, I then (on the LHS page) write up a complete microphone/console channel number plot --- I also write up on a separate log sheet an idea of what I consider could be any EQ/compression/reverb/etc that is required for each performer (and/or song).
At this stage I have not even sat behind the console.
Doing a "professional" production (and a school concert is a professional performance as far as the parents are concerned and remember -- every parent wants to hear their child perfectly --- in a cast of many on stage as far as Mr/Mrs Jones are concerned, there is only one person on stage and they are there to hear that "solo" performer!!!!!) is all about ensuring that everyone on stage can be heard at the correct volume.
The comment passed a few times above that professional performers do not require amplification is not strictly correct today. You go and watch a professional production of (say) Phantom, Sound of Music, King and I, Lion king, etc, virtually every performer is wearing a microphone (or is being amplified by some means). I have done the FOH sound for some of the biggest opera performances featuring some of the most famous opera singers and I have used up to 80 mics --- the trick here is for the mics not to be visible and for the audience not to realise that the production is being fully amplified (as mentioned earlier sound reinforced rather that in your face amplification).
To the initial poster, I am sorry, but to get a satisfactory result, you will need very good (is suitable) equipment (ie correct microphones for EACH production, a very good desk (have a look at the Behringer X32), good amplifier/loudspeaker combination (and for these to be mounted and tuned correctly) and ideally a venue that has been acoustically treated for live theatre with microphones --- NOT the school basketball arena with a make-shift stage at the end, polished timber/concrete floors and concrete walls.
You also did not mention the age of the performers. When doing the local dance academies with performers ranging from 5 - 18 years of age and with greatly different volume levels, I often have to place a microphone (ie taped and then make-up applied to cover the microphone/cable) less than 1" from the performer's mouth --- sure I have the correct mics for this --- with the ones I often use I can purchase for about $US20 (that's correct!!!) and they perform as good as or better than DPA's, Countrymen, etc all of which cost many hundreds and all of which break just as easily when trodden on by performers -- a very common occurrence!!!!!
Finally, for every show/performance, and irrespective of how much level setting, microphone dsp, etc, is applied and irrespective of how-many rehearsals and how good the performer/s is/are, EVERY performance will have to be mixed by a competent sound mixing engineer if you want to achieve a first class result ever performance. If this were not the case wouldn't every live theatre/venue and every major touring act, simply have a sound Op present at the rehearsals to set the microphone levels/Eq/etc and then be dismissed once the actual performances commence --- NO!!! the Op is there for every performance because performers perform differently and with different volumes and tone every night.
Just some food for thought and after almost 50 years of doing FOH at all levels and performance types and with all forms of equipment and in all forms of venue, I am still learning and/or finding a new challenge at every performance and venue.
David