I don't know why you people point out the fact that "because females".
In my opinion, it doesn't matter if it is two females -> 16 year old guy, or two males -> 16 year old girl; just like it doesn't matter whether it's two females and a young lady, or two males and a male student.
Ethics of Teacher/student aside (Just fire the teachers for sleeping with their student; that's the biggest punishment they should receive), it is legal to have sex with 16-year-old as long as everyone gives consent. They had it? Then what's the problem?