Since you are teaching I assume you do know all I am going to say. However after reading posts I've decided to leave a couple of notes.
Before the toss put a racquet inside of baseline next to the toe of the left foot. The head of the racquet is pointing toward the net and a little bit to the right (10 degress) The ball has to land on the head of a racquet after the toss.
Prior to a toss
Holding the ball conveniently with all fingers. Ball located in the middle of the palm. Arm is relaxed and all the way down (a bit bended).
For advanced(beginers) - imagine the position of the ball in the air at the point of contact with racquet..
During the toss:
The motion is slow and smooth.
The left arm(throwing the ball) goes all the way up and stays straight up, just like you are trying to reach the ball.
Position of feet, work of right arm, head and ect. are not included