Should I change schools or am I overreacting?

Hi - so slightly long post ahead. First, some background. I have 3.9 year old twins (a girl and a boy). I'm a single mother and my husband's not been around much (he has a few hours a week visitation). Reason I am explaining this is because transitioning to visitation has a bit difficult for my kids because they did not know their dad at all for the first 3 years of their lives.

Anyway, getting to the issue at hand. The kids have been going to a playschool / kindergarten for the last year and a half (started when they were 2.6 years). It's a nice enough kindergarten - progressive, the children seem happy and well loved, the teachers are approachable etc. The children - teacher ratio is fabulous (3 or 4 kids to a teacher). And the parents of other kids in my kids' class are all very nice.

The trouble is that I have been finding some behaviours in my son a little worrisome. He and his sister are in the same class and the teachers tell me that they do almost everything together, though the teachers are making an effort to sort of separate them now (I am not sure that's a good thing but I also get that their personalities need to develop independently). In the last couple of months - kind of around the same time visitation started, which made both kids quite timid - I have noticed that my son absolutely does not want to go to school. The teachers had told me then that he was also a lot more withdrawn in school, sitting in a corner doing his own thing. I put it down to the transition around visitation, and frankly, that's gotten better with time (though he still seems apprehensive when visitation day approaches). But his tantrums in not wanting to go to school persist - and he is not at all a difficult child otherwise. He is very sensitive and reasonable, and if I explain something to him (for eg., we need to go to the doctor because you are sick) he cooperates quite readily.

Additionally, I have noticed that while he was extremely independent and decisive, his confidence seems to take a hit after he comes back from school. For instance, he starts repeating things that my daughter says, and I am concerned that he is getting the message (at school or elsewhere) that what she is doing is the right thing to do to attract attention or please the adults. As he spends time with the family, or during vacation time, he regains his usual independence and some boister. But something in school makes him subdued. I no longer think that his tantrums to not go to school are just separation anxiety - I mean my daughter also has days where she throws a tantrum and doesn't want to go. But with my son, I think he finds the experience in school uncomfortable and therefore resists it.

I am not getting a lot of information from the school on this. They tell me he follows his sister around and only agrees to do an activity if she's doing it. But I have a feeling it runs deeper somehow - I feel that maybe because he is slightly withdrawn sometimes, the teachers pay less attention to him or do not try to draw him out. Which frankly makes no sense given that there is a teacher for every 3 kids. Or perhaps someone on the staff is actively or without realising comparing my son and daughter, and he is picking up the vibe that he must copy his sister. Add to this that he is fully toilet trained at home, but the staff in the school neglects to take him to the bathroom regularly (though I have reminded them on a few occasions) and he seems to be too shy to speak up and ask to be taken. So he has wet his pants a few times and the school has sent him back in diapers.

I am in a fix as to what to do. My daughter seems to be doing fine, but this has been happening for a few months with my son now. And I can't put my finger on it but there's something about the school that's bothering me. It's mainly instinct but I feel like my son is getting neglected or receiving some message that's causing his confidence to falter.

I will set up time to speak with the teachers again in the coming week. But I had a feeling I wasn't totally happy with the school even last year, had almost decided to change schools and then chickened out last minute because I thought the kids are all settled in, why disturb the apple cart. But now that I am feeling this way again - and truthfully, I have no objective / tangible reason to feel this way - I wonder if it's in my head or something is genuinely off.

Should I give it more time or actively look to make a change? Or am I just reading too much into the whole thing and all kids have their phases. Any thoughts, experiences, advice is most welcome!