you are certainly not alone in having your little one struggle in preschool. Lots of young children have trouble adjusting. The problem lies when they communicate their feelings of frustration through aggressive behavior. Keep in mind that little kids do find it easier to communicate their feelings with actions than with words. Her behavior is sending a clear signal that she is not happy and probably overwhelmed.
Though people talk about the 'terrible twos', I found with my own children and in my practice that three can be far more of a challenge. Three year olds may still resort to aggression when they feel upset, and the trick is to teach them other ways of handling their problems. Little children can and do learn better ways of managing their angry feelings, but it may take direct teaching and support to make that happen.
As I always recommend to people, the first stop should be to your pediatrician. There are many everyday medical problems that make children irritable. For example: ear infections (or fluid behind the ears), food allergies, constipation, growing pains, erupting molars, and other physical maladies. Think also about how she is eating (healthy foods promote good stable moods) and sleeping. She should be getting at least 10 hrs per night if she is going to feel good the next day. Kids can have physical problems that they can't tell us about, so best to make sure you rule out the potential physical causes first.
You have made an important observation, that there is a specific set of triggers for this behavior at school. Whatever these triggers are, they are not happening at home. If you can figure out what is triggering the behavior at school, you will have the power to target the unwanted behavior for what behaviorists call 'extinction' (when a behavior no longer occurs). So from a behavioral standpoint, your therapist is absolutely correct. His/her interpretation of the situation is NOT offering an excuse or suggesting you should tolerate the behavior. We don't want to underestimate how stressful a poor school fit can be for a child. But you are both correct that there is something about school that is overwhelming for your daughter. My guess is tactics like time out (which is often misused in a punitive way) may be making the problem worse.
I really appreciate that you are trying to teach your daughter right from wrong. I know its much harder than ignoring rude behavior as so may parents seem to do! But keep in mind that punishment is not actually a very effective means of changing behavior. Decades of research show this. And with young children, very small, short punishments are the way to go. They are more effective than big punishments like taking all the toys away. Big punishments tend to increase angry and frustrated feelings. They can make the child more likely to act out again because they feel so angry about being punished. Since your plan is to help her be the wonderful kid she is at home, using techniques that will make her more frustrated are not likely to make her feel like being sweet. Besides, it is stressful for a little child to have a younger sibling, and you don't want her taking out angry feelings on the littler one, or imagining that she is the 'bad' one while her sister is 'good.'
The key to changing behavior in little children is to figure out what is setting her off. Sometimes you have to be an outsider (like the therapist) to figure this out. Have you considered asking the therapist to do an observation of your daughter in the school? This would yield very valuable information. An objective observer would be able to determine what the problems are by noting what specific situations lead to aggressive behavior. Psychologist and therapists are trained to conduct formal observations, so ask a qualified clinician. Here are some of the common causes of childhood frustration in preschool: being overstimulated, a classroom program that is not well designed for young children, not enough opportunities to move around, poor discipline practices in the classroom (like punitive time-out or shaming), poorly trained classroom staff, and negative peer models.
After studying what is happening in the classroom, the therapist can then suggest a program that WOULD be a good fit, or offer suggestions to the present program to help them change to meet your daughter's needs. For example, perhaps they could respond to rude behavior by ignoring it instead of giving her more attention for it.
I believe that kids really DO care and want to please adults. You have seen this quality in your daughter. In a situation where she CAN please adults, she does! She's a good kid at heart. There may be something about her that makes it harder for her to cope with preschool as compared to other kids. She may not have skills to deal with interpersonal conflict. She may need to nap sooner or eat more frequently. She may have a learning disability or attention problem that is showing up early. But this does not sound like a bad or malicious child at all--she sounds like she is struggling!
Consider going back to the last therapist (or find another one) and ask for a school observation. Then meet to talk about what he/she observed. Your therapist can then create a plan for changing the behavior that can be implemented at home and school. These behavior plans may seem like they could not possibly work as well as harsh punishments--but if you give it a real trial you will find that not only do they work better but you will FEEL so much better as a parent. Proper behavior plans are designed based on decades of research about how to change human behavior. They work very well and relatively fast with young children if you can commit to them. Its definitely less of a pain in the neck than shopping around for a new preschool.Check out my article on my profile page about behavior plans so you can learn what I am talking about. Remember, an effective behavior plan is designed to teach and increase the use of new skills, not punish.
The therapist may refer you for a psychological assessment if he/she observes early signs of learning problems. He/she will probably also want to work with you for parent coaching, and spend some one to one time teaching your daughter how to handle situations like fighting over a toy or being called a rude name.
There are many great books to check out (like I said, you are not alone): How to Talk so Kids will Listen and Listen so Kids will Talk by Adele Faber
The Kazdin Method by Alan Kazdin
The Explosive Child by Ross Greene, Lost at School by Ross Greene
Disclaimer: this post was written for informational purposes only. It is not intended to replace face to face medical or psychological care. It is not intended to create a patient-clinician relationship, nor to give or rule-out a diagnosis.