In general for good audio, you have to get the microphone close to the sound, even for directional microphones such as shotgun microphones. Shotgun microphones don't zoom in on the sound in the way that a zoom lens does. Instead, they tend to exclude side sounds.
If you just set up a camcorder on a tripod with a wide shot of the entire classroom, including you and your students, and use the built in mic on the camcorder, the sound will not be very good. However, it might be usable for basic purposes, if everyone talks loud. If you want better sound, you will need to move the camcorder closer to each person speaking as they speak, or you will need to have an external wired mic with the camcorder that is placed close to each person as they speak.
Trying to record audio from many people at various locations in a classroom is non-trivial to do reasonably well. Don't expect miracles just because some camcorder is claimed to be good at audio. The basic physics of microphones just doesn't allow that.
Perhaps you could describe more thoroughly the scenario you are trying to record. For example, if you only need to record your own voice, then a lavalier microphone on your lapel wired to the camcorder would be the way to go, but it would be poor choice if you are also trying to record what students are saying.