As you didn’t mention your daughter’s age, it’s difficult to answer your question. I’m going to guess that she is around 3and write accordingly. But, whatever her age, I wouldn’t worry too much about it. It is far more important during this developmental period to talk than to talk correctly. She may have heard younger children in her group speak this way, or she may have enjoyed playing a “baby” role in dramatic play in her group. I would suggest that you correct her quite casually and try not to make a big deal out of it. If she hears you and her father say, “I want” she will begin to talk that way regardless of what you say or do.
I would talk to her teacher about it but not in terms of why your daughter “is not corrected in school.” You may learn that she does indeed correct your daughter and that the other children have picked up this habit from her! That is, there may be something in the situation that makes your daughter want to be a baby. On the other hand, she might tell you that she doesn’t talk that way at school. In either case (or if there’s something I have not anticipated), it would be a help to talk to her teacher in a non-accusatory fashion. Incidentally, you didn’t give any information about the composition of your family, but this kind of regressive language is common after the birth of a younger brother or sister. Is there a new baby in the picture who makes it seem desirable for your daughter to be a baby?
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.