It was only when the 11 year old tried to commit suicide and ended up in the hospital that his parents realized that their son was depressed because of a failed friendship. A few of his friends and even his teacher knew about it. But, it was only in the end that his parents understood his real issues. Often, it is only in the end that most parents realize that their kids had been going through severe mental stress or distress.
When a child is under mental stress, the parents and the teacher could observe him/her carefully. You could observe whether the kid is uninterested in studies or whether he/she likes to be aloof and lonely or even sit inside their rooms with no interest to play with peers.
If you realize that your kid is indeed under some sort of stress, then you should develop a kind heart to understand his/her problems. Their problems may seem trivial to you, but you should listen to your children without blaming them. You should make them understand that you are with them and are willing to listen to their problems. The parents should develop an intimate relationship with their kids so that they would feel comfortable opening up to you.