Should Lay Buddhists Teach the Dhamma? Should a lay Buddhist teach the Dhamma in a dhamma centre? Can a lay person teach just as a monk does? If so, are there any differences (e.g., should they sit on a different level, should the audience pay respect in a different way, etc.)?
Lay practitioner ==> monk is not a one way street. Some monks renounce their vows for various reasons (not always bad). A famous example in Western Tibetan Buddhism is Chogyam Trungpa Rinpoche who ordained in 1947 and renounced in 1970, but kept teaching in lay status all the way until 1987. He still sat on a high platform and enjoyed the same honors as an ordained teacher. This is less about your official status and more about your spiritual achievements and realizations.

As Buddha himself said (AN 5.166),


  Why should an elder be honored, respected, held in esteem, and venerated? On account of his broken teeth, grey hair, and wrinkled skin?  


...  


  Possessing five qualities, bhikkhus, an elder is respected and esteemed:
  
  
  He is virtuous; he dwells restrained by the Patimokkha, possessed of good conduct and resort, seeing danger in minute faults. Having undertaken the training rules, he trains in them.
  He has learned much, remembers what he has learned, and accumulates what he has learned. Those teachings that are good in the beginning, good in the middle, and good in the end, with the right meaning and phrasing, which proclaim the perfectly complete and pure spiritual life -- such teachings as these he has learned much of, retained in mind, recited verbally, mentally investigated, and penetrated well by view.
  He is a good speaker with a good delivery; he is gifted with speech that is polished, clear, articulate, expressive of the meaning.
  He gains at will, without trouble or difficulty, the four jhanas that constitute the higher mind and are pleasant dwellings in this very life.
  With the destruction of the taints, he has realized for himself with direct knowledge, in this very life, the taintless liberation of mind, liberation by wisdom, and having entered upon it, he dwells in it.
  


From this we can see that it is not whether you are a lay person or a monk that is important for you being a teacher, but your spiritual achievements and realizations, plus communication skills.