As others have mentioned, your example is perfectly fine. However both "loner" and "isolate" are harsh terms that many teachers might refrain from using, because it implies extreme behavior. Instead they can soften their advice to the parents with something like:
Your child seems to have trouble making friends.
Your child seems not to have many (or any) friends.
Your child seems to prefer to play by himself.
Your child keeps to himself most of the time and doesn't seem to want to join the other children.
Your child seems to prefer his own company and doesn't like to play with the other children.
Your child doesn't seem to want to participate in group activities with the other children.
And so on. Notice I use "seem to" and "prefer" to indicate it's my personal observation or impression and not necessarily an objective fact. Even if it is true, teachers know that parents can be sensitive about their children and couch their advice in careful language to avoid confrontation or even legal repercussions.
Additionally the language focuses on the behavior and not the cause of the behavior. Saying a child is a "loner" implies that the child's fundamental personality is to want to be alone. But the behavior could be due to any number of external factors, from bullying, to developmental issues, or even abuse of some kind. It's safer (and more accurate) for the teacher to talk about what she has actually observed and not jump to any conclusions.