I'm going to do something risky: I'm going to my own ideas as to why, it seems, we're hearing about more and more of those relationships. As I am not a researcher in any field related to the phenomenon, please take what I am about to say as opinion that has been shaped by my own observations and experience, and nothing more--or less.
Some people say that we're hearing more about such cases because they're being reported more frequently. That's probably true, and some people who were sexually exploited years, or even decades, ago are coming forward, much as many men who, as boys, were sexually abused by priests are now speaking out. However, I don't think increasing openness about the topic and reporting of incidents can account for all of the sad cases we're hearing about.
I have long believed that most elementary- and high-school teachers begin their careers much too young. Typically, they start when they finish their bachelor's or master's degrees, depending on where and what they're teaching. If they have followed the conventional trajectory, they are in their early- to mid- 20's when they finish their bachelor's degrees, and their mid- to late- 20's when they finish their masters'. Therefore, they are--especially if they are teaching high school--not that much older than the kids they're teaching.
Now, some would argue that young people are more knowledgeable about sex and relationships than earlier generations (which include yours truly) were at a similar age. More information is available more readily, to be sure. But more information doesn't necessarily translate into more emotional development. Nor does hormonal activity which, according to some reports, is occurring at earlier ages than it has in the past.
These neophyte teachers, who are just barely out of adolescence, spend their days among adolescents and young children. That can be a trap for young teachers who haven't fully learned what's appropriate, much less nurturing, behavior with teenagers and children. Some of those teachers were, like many of their peers, engaging in indiscriminate sex (along with binge drinking and other kinds of reckless behavior) only months, or even weeks, earlier, when they were in college.
Of course, being among other teachers and school employees and administrators who haven't matured emotionally doesn't help them, either.
These problems have existed for a long time, you say. So why are there more cases of children who are sexually exploited at school?, you might ask.
Well, I think that as families and other structures that were supposed to protect kids are breaking down, more new teachers are coming from what might be called "dysfunctional" families and communities. A kid is more likely to be abused in such an environment. Even if he or she is not abused at home or by a family member, he or she is more likely to experience sexual exploitation, or other forms of violence, elsewhere.
While what I've described may be more common among the poorer and less-educated, no economic or social class is immune to breakdown and dysfunction. Even in so-called intact families, kids may not spend very much time with their parents, or other responsible adults who have their best interests at heart. Hence, important lessons about social and sexual mores, not to mention the ability to know when one is in danger, is not passed down.
Furthermore, most young teachers have never been in any kind of professional environment but school. Therefore, they do not learn, as they might in other environments, that they are responsible for their actions and that those actions have repercussions. That is why some teachers and school employees who are caught having sexual relationships with kids--and, worse, exclaiming that those kids are the loves of their lives on Facebook--don't understand what's wrong about what they've done. And some, like Mary Kay LeTourneau, think that they are being deprived of their "right" to make their livings as teachers when they are imprisoned and banished from the profession.
In brief, I think we are going to hear about more teachers having sexual relations with their students because they are less stable and emotionally mature than teachers of previous generations. And even those teachers were too young, spiritually and emotionally, to be teaching kids.