Sure your child may have had experience in a group setting, but the experience she is having now may be radically different from anything she experienced before
What was the setting like at preschool? Smaller group size (10 or less kids) or lots of children? Unstructured play or more like a Montessori? At someone's home or in an actual center? Full curriculum with significant time spent reading, writing, doing math and so on?
If her experience was unstructured and more play than desk-time and listening to a teacher giving direction and lectures, she could be having a very difficult time getting used to the concept of following directions, having to sit quietly for long periods of time, and actually having to accomplish assignments.
A larger group of kids (18 is a lot!) could be overwhelming and just too much stimulus if she was always with a small group of kids. If this is a fulltime kindergarten, these days, that is pretty much the same as 1st grade when we were children. That means longer time spent at desks working on actual curriculum, and whole lot less play and free time.
If she's not ready or used to this, she might just be having a hard time adjusting to the change.
For now, work with the teacher and see how her approach to getting the kids used to structure works out. Perhaps in a few weeks or more, your daughter will start conforming.
Right now it's too soon to see how the teacher's approach will jive with your daughter's temperament and learning style. By mid year, you'll know if you may need to start considering alternative environments for your child's learning style and personal needs.
There are some children who will never do well in a very structured setting. It's all about the way they are wired. Traditional schooling isn't very doable for kids with short attention spans, or those with sensory issues who are very sensitive to loud noises, too many people, too much stimuli, and even too much work and not enough breaks or time for movement.
Then you may want to consider a private school where the class sizes are smaller, or homeschooling where the environment and curriculum can be tailored to her learning style.
But for now, it's too soon to tell. Give the teacher and your daughter a chance and take it from there. At home, make sure your daughter has time to decompress from the noise and activities of the day. It sounds like she's working out stress from the day. If she's naturally a high energy kid, sitting still, focusing on assignments and having to deal with tons of kids is probably overwhelming and she's bottling it all up so that when she gets home she's unleashing everything on the family.
Maybe a warm bath, gentle music, a snack, time to vent and talk about the day with just you and her is what she needs to decompress. Maybe talk with the teacher too, to find out what the average day is like. Maybe there are things that can be done during the day to help her find coping strategies as well.
Other things to consider; what is she eating at lunch? Does she get enough time to eat? Food is a huge factor in child behavior. These days kids do not get enough time to eat lunch and wind up going hungry, or sometimes get lousy highly processed lunch from school etc.
What about naps? Do they get them or is it a full day straight through? If she's used to naps or a significant rest but now is expected to sit quietly most of the day, this can be a shock to the system as well.
Many kids have to wake up and start school earlier than when they attended preschool. Is her day longer now? Is she lacking enough sleep that she used to have before?
Is there someone at school bullying her? What abou the teacher? You don't want to think bad things about the teach but it can happen...a teacher who doesn't click with your child and vice versa. Only way to find out is to talk with your daughter.
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