I relate so much to you. I'm not at all a shy person. I can talk and talk and talk for hours to some people. But, in some situations, I get terrified if I have to talk at all. I didn't used to be that way when I was young. I was the first person to volunteer to give a speech, read aloud, etc. When I got older, I developed panic disorder triggered only by certain social situations.
I'm currently unemployed because of this problem. I don't understand what brought it all about over the past few years. Even if I run into people who make me really uncomfortable, this panicky feeling sometimes comes over me.
I've had panic attacks in social situations like you described (classrooms, etc). To others, they really didn't even notice, or thought I was just tired or out of it.
I believe it's just the fear of the fear. We've trained ourselves somehow to become panicky over something that is so silly. It's like allowing your anxiety to drive you crazy. I never get this way but in situations where I have to talk, but I'm not a shy person. Go figure!
I've also taken beta blockers (metrprononal) to avoid panick in these type of situations, but the doctor decided that I shouldn't take them all the time for panic disorder. That's the only medicine that ever worked for me. As you say, it's a crazy "adrenaline rush", and you have no control. It just strikes and leaves you paralyzed. But nothing stops it for me except distraction or beta blockers.
Serendym (natural herb combination) has helped me a little bit with social situations that make me not quite as nervous.
Stay in touch and take care.