In Being the Boss, by Linda Hill and Kent Linebeck, the authors make no bones about this dilemma: a boss must not and cannot be friends with subordinates. Why not? A friendship can cloud a manager's judgment when it's time to make a tough decision. A friendship can allow both the manager and the subordinate too become too comfortable, too at ease and too relaxed, thus leading to poorer performance as well as an inability to be honest with one another about performance and/or expectations.
As much as it pains me to agree with the authors, I have to agree with them on this point. Interestingly, the authors provide no advice on how to accomplish this. The trick is to build relationships in which subordinates feel the manager cares for their personal and professional well-being without getting wrapped up in their lives and personal affairs. I'm not sure there is a manual for this. Based on my experience, I believe this requires judgment and discernment on the part of the manager. I believe it is the manager's responsibility to know where the line is with each individual and to gauge the appropriate depth of the relationship with each. One of the most tempting pitfalls managers must avoid is sharing confidential information with, venting to (about work) and confiding in subordinates. This can be especially tough for educators in leadership positions.
The bottom line is this: no matter how much an educational leader depends on relationships to be an effective manager/leader, the line between him and subordinates must remain intact, and he must work constantly to build strong relationships that don't morph into friendships.