To be honest, this story comes across as a bit sensational. Two minutes of research shows an *out* from the wire taping statute.
Necessity is a defense, although quite tough to use in practice; it's a bit like successfully using an insanity defense -- possible, but highly unlikely. Also, the necessity description you provide is a general statement of the principle, not the language Pennsylvania has adopted. As a common law defense, the state courts adoption is what controls. Moreover, necessity isn't always a defense (even if you prove the elements) -- it depends upon how the statue is written.
Turning to the OUT I mentioned above, there is an exception built right into the statue. Full text can be found here:
In relevant part, the wiretapping statute provides:
5703 Provides "**Except** as otherwise provided in this chapter, a person is guilty of a felony of the third degree if he: (1) intentionally intercepts, endeavors to intercept, or procures any other person to intercept or endeavor to intercept any wire, electronic or oral communication;"
5704 Contains a long list of exceptions. For the most part they apply to police, telecom, or telemarketers (go figure). Subsection 17 is relevant here ...
5704 (17) Any victim ... to intercept the contents of any wire, electronic or oral communication, if that person is under a reasonable suspicion that the intercepted party is committing, about to commit or has committed a crime of violence and there is reason to believe that evidence of the crime of violence may be obtained from the interception.
If the bullying was as bad as the article describes, the student could surely have reasonable suspicion that the party was about to commit a crime of violence.
You can read more about this here: