Staff issues at work

I am the head of my own small but very successful, private pre-school. Recently I employed 3 new staff members ensuring that they are assisted every step of the way in understanding how things get done with the children, making sure that parents should have nothing to complain about in how their child is being cared for or educated. The one main reason for the school being successful is due to parents commenting on the care their child receives and high standard of education that I've worked hard to maintain over the years. However I find that slowly but surely the new staff tend to get lazy about carrying out some duties. When I take it up with them in private they are apologetic and understanding, however when I have staff meetings with the 3 of them, I often find they display an indifferent attitude, especially if it concerns an issue about a parent complaining, or how they may have the incorrect method of teaching an activity. It isn't easy to find pre-school staff in my area who are trained in this particular field of education ( Montessori ) so I can't just get rid of them, also this would be a black mark against my school ( high staff turnover etc...) Each one of them started off very well, but as they became closer over the months, their whispered conversations appear directed at me. I feel undermined but am powerless to stop their negative behaviour even though I always maintain courtesy and a positive attitude towards all 3 of them. How do I deal with them shirking certain duties but obviously complaining to each other when I take it up with them?

EEkkk - do these 3 staff members have a connection with one and other ?either way thta doesnt really affect my advice though it may strengthen it if they do. Give all 3 a written warning listing complaints concerns and all the duties in their roles they are forfilling. Then should they not change either issue a second more strongly worded warning or simply reduce the hours of one (or sack) it will send the message of the other 2 of i am not playing nice anymore this is a buisness and a school and dead weight will be cut . If they talk to gether they will all know they are on warning they will each know that they are at risk in this economy people want thier jobs as an employer you have the power ! use it !! and good luck being tough isn't nice but sometimes it is important to do .