You can adjust the local latency compensation also during the session.

A manually calibration works like this:

1. Switch the playdirection to the student 
2. Open the Metronome and switch on "remote control" in the metronome
3. Let the student clap to the metronome
4. Say the student to rise the latency compensation until the clapping is in sync to the mixer

Tipp: When you use the EarPods of Apple on iPad the latency should be around 0,4s - Try this value.