getting the best performance
Getting the best out of an inexperienced cast is tricky, but play to their strengths. Occasionally a timid child can rise to playing the 'very angry ogre'. But usually for naturalistic performances, playing to type is easier.
Don't push them to 'act'- let it flow whenever possible.
With very young children get them to say their lines after you say them, and encourage them to mimic your actions
rehearsals
Drama
Learn lines as soon as the script is ready, this can waste precious time on set. Small groups in a class situation is ideal. Rehearsals and blocking can also be done before filming starts, and if the camera crew know all the action they can take responsibility for how shots are composed.
Documentary
Rehearsals are still useful even if you're interviewing class members. Although rehearse using a different subject, to get them used to speaking in front of camera, but so they are not parrot-like with their actual interview
other points
Some of your group may be unhappy to be in front of the camera - be aware of these politics.
If convenient, actors who are not in shot should stay in their positions to help those who are performing to look in the right direction.
If your team is small, actors can also be involved with filmmaking process, but they need to be clear which role they are doing each day.