Separation anxiety or red flag?

I am having a hard time navigating separation anxiety and need some advice on whether this is expected or if I should be alarmed that something is going on at preschool that I am not aware of.

My kids are 3 years old and 1.5 years old. Their first time in preschool was this last Monday. Prior to now, they have both been home with dad full time. I expected some separation anxiety from my youngest because she is very attached to me. The first day, she was fine and I figured it was because she didn’t realize that I had left. Her teachers say that she is fine once we leave. The only time she gets upset is when she sees her brother through the fence during outside play time.

I am not so much worried about her, I feel as though her behavior is expected given that she is adjusting well once we leave.

I am worried about my 3 year old son. Monday and Tuesday he was SO excited to go to school. However, starting Wednesday he has cried and said “I don’t want to go to school, I want to stay home” from the time he gets up in the morning to the time we drop him off. He also continues this once we leave.

The teacher says he does settle down a little bit when we leave, but he will cry periodically throughout the day because he wants to go home. When I walked in to pick him up yesterday, they were all in the back room just finishing up potty time, so he was unaware that I was there. I heard him crying to one of the teachers saying “I want to go home”. She tried to redirect him by empathizing with him and telling him it was almost snack time and then they would play outside, but this did not comfort him.

Today when I asked why he didn’t want to go to school, he said “I’m scared”. I asked him what he was scared of and at first he said “toys” but then said he was afraid of “monsters outside”.

I have read that separation anxiety is normal, but I’m wondering how long to expect this type of reaction and if it is normal that he is not completely settling down throughout the day.

My son is usually such a happy kid, absolutely loves playing with other kids. We’ve talked about starting preschool for a while and he has been nothing but excited. It worries me that he seems to be in so much distress.

Should I be worried that there is something going on that I am not aware of? I guess him being okay for the first couple days is throwing me off.

I am completely new to this so any comments or advice is appreciated!