Well, I've had teachers who do use phrases like "Do you want to get an F?!"... That's nothing new, teacher's have been doing that since the dawn of time, when they're ticked with students who could do better, but aren't putting forth an effort (I was one of them!)

Now, I don't know what you were talking about when you mentioned your friend... Those events were a little hard to follow, I think you need to elaborate a bit more clearly...

You mentioned your friend didn't feel like talking (did he refuse to respond to the teacher in class??), then got into trouble (for what?), then e-mailed the teacher (about what??), then was threatened the at the beginning of class (the next day??) for not talking...

Now, I will say that threatening to kick out someone who is just a quiet person who doesn't raise his hand much is completely uncalled for behavior... (Unless your friend was instructed to solve a problem or somesuch and refused.. Again, don't know the specifics..)

Ultimately, that kind of getting in your friend's face, if he just doesn't frequently volunteer to answer things in class, is out of line, and you should talk to a guidance councilor or even an administrative person about this... (Not that they'll necessarily do anything, besides pat you on the head & say "don't worry, it'll be ok..."