In most cases admins are very busy and simply want to accomplish that the letter results in the person stopping breaking a centre rule. This might be not smoking on the centre grounds, making noise so that other people cant meditate or wearing skimpy clothing when it is a community of monks & nuns practicing celibacy.
There are 2 scenarios:
1) the admin is asking someone to stop breaking a centre rule
2) the admin is being asked to deal with a dispute between two people
If you are results orientated before you write the letter you have to work on what words will accomplish the reaction out of the student that you are seeking. The reaction you are seeking is positive, supportive, sympathetic and cooperative. Not rebellious, weepy, angry, defensive. Its easy after working all day and receiving criticism to just type out a quick 'keep the rules or else', print and send. But this is not saving you energy in reality. Because when they generate the wrong response then you will have all that additional aggro to deal with.
If you are an admin therefore its worth putting some time and thought into good letterwriting and communication skills as it will pay off over the years that you are doing the job and lead to your community becoming very harmonious and successful.
Since admins are busy let us write some such letters here as samples. Perhaps the letters will be so well written and versatile that they can be copied/pasted and used.
I was also planning to write more detailed guide to good letter writing at some point. Ideas are welcome.