Sorry - but there is an error in the second sentence - it should read -
So, children suffering from sm (the shortform for selective mutism) will be mute only when they feel UNSAFE or uncomfortable, so it is the situation or environment which determines the beahviour (or muteness) of the child.
Children who suffer from severe anxiety use coping mechanisms to help them manage their fears - mutism is one of these behaviours. So, children suffering from sm (the shortform for selective mutism) will be mute only when they feel safe or uncomfortable, so it is the situation or environment which determines the beahviour (or muteness) of the child. When the child feels safe and comfortable (as in the home), then speech is normal.
Clinging and hiding behind a parent's back is a behaviour common to children suffering from anxiety. However, this behaviour does not mean the child is suffering from an anxiety disorder; just that there are some anxiety issues. And, often this behaviour is seen in children who tend to be shy or more reserved.
Children usually learn how to manage their fears and stresses when they are shy and/or reserved. Often parents will claim their child "outgrew" this phase when, in fact, the child "learned" how to manage his/her fears. Children who suffer from an anxiety disorder usually are not able to manage their fears and/or stresses without treatment (and this does not mean medication is always involved). Children with anxiety disorders do not outgrow their anxiety nor does it go away.
So in order to answer your question, we need more information. How often is your daughter mute? Are their other behaviours that are troubling as sleeping, eating, or toileting issues, social problems, tantrums and/or frustrations, etc. And, exactly, what do you mean when she is "doing well at school" - academic or social? We hope to hear from you soon ....