How did Mason end up at BHHS? So, Liam transferred to BHHS because of anger issues. But, Mason is his best friend since elementary school. How did Mason end up at BHHS? Mason seems like he knows everything about Liam's history so did he transfer over to BHHS with Liam or was he already at BHHS before Liam transferred?

It's never explained AFAIK, like most things in S4.