Other forms: responders
A responder is someone who answers a question or who acts quickly in response to some event. If there's only one responder to all of your teacher's questions, he'll start calling on other students to answer as well.
Anyone who responds, or reacts, in some way is a responder, whether their responses are spoken, sung, or acted out physically. You're likely to find this word in the context of "first responders," a term that's used for police officers, firefighters, EMTs, or anyone else whose job puts them among the people who help in an emergency or disaster.