All principals who really care about the welfare of teachers have been proved to be well-managed principals; and those who really care about the welfare of teachers firstly focus on solving the housing of young and middle-aged teachers. 
All principals who first focused on housing for young and middle-aged teachers were principals who managed well.
 those who do not first focus on solving the problem. The principals in the housing of young and middle-aged teachers are not those who manage well.