I am a pre-k Montessori teacher, I have a mixed class of 21 3 and 4 year olds. My children do not need to clean up all of their work, just put name tags on their lessons and they can finish it later. However, if we transition from our work time to our group time we use a song. Most teachers use and instrumental song (I use Enya's Tea-House Moon). When the song starts, it is a signal that it is time to stop working and either get their name tags, or clean up their lessons. The children know they must be seated in their places by the end of the song. This works very well.