What I think is that they've split the boys into groups of four. The first four will be in the first 24 hours and these will be the boys who have best physical and mental condition, and who have taken best to going underwater with the mask and maintaining breathing and calmness. And then the next group have 24 hours of extra nourishment, a bit more practise, and rest periods. And then the next four. And then the coach and the remaining medic-divers who would stay behind until the last of the Mae Sai 13 are safely out... and worst case scenario they stay a bit longer with them.
JMO.