Don't know what to do

HI, I'm a mother of two and for me safety of my children is a big thing. Well my children have been going to a public school now, my daughter 4 years and son just this year, well the problem I have is this:

1. The first incident that I can recall was, on the first day my son was to attend kindergarten, my husband had walked him up to the front doors of the school. After which, my daughter who was at recess at the time noticed he had pulled up and you know how kids are got all excited and decided to go talk to her brother, well she walked out of the playground and over to the front of the school, which my husband wasn't too happy about and scolded her for it of course, however he did notice also that not one teacher that was supposed to be keeping an eye on the children even noticed she had walked away. So we just went ahead and left that issue alone, spoke to our daughter and told her not to do it again. Our daughter has autism so gets excited rather easy and can get sidetracked so you have to keep track of her.
2. The next issue that came up was that my son who is 5 years old, had gotten into some altercation with another boy right at school dismissal. Well my husband was on his way to pick him up when he ended up finding our son about 2 blocks away lost not knowing where to go. When he saw his dad he went running into traffic to get to him, which had my husband not noticed him I don't know what would have happened. Anyways when we talked to our son about why he had left school, he told us that one of the teachers had split up the argument and had told him to walk home. When my husband confronted the teacher she said she thought he was a 3rd grader, which if you saw my son you'd know that was a complete lie. This all happened at from the time my husband found him at 3:04 to about 3:15 so right after school. My husband usually leaves the house at 2:55 because of the massive amount of traffic in front of the school.
So after I found out about all of this I did what every parent would do I went to the principle to ask her why a teacher had sent my 5 year old son home to walk in the rain no less when I specified to them already I wanted him to wait for us in front of the school for pickup with his autistic sister, who was very histerical I might add after this whole incident.
Well the principle preceeded to tell us that oh well nothing bad happened to him so theres nothing I can do. Then she had the gall to scold my son in front of us about not walking off the premises. Which to a 5 year old is confusing because he was told by a teacher to leave the school and your supposed to listen to the teachers, which was pretty much his reply.
So after that I tried calling the school board to make a complaint about it. They did everything they could possibly do from telling me that the superintendant is too busy for these matters to maybe I should just talk to the principle again instead of making a long drawn out complaint over the matter.

So now after all of this, the teachers still don't watch my children like they should, we've gotten non-stop harrassment from the school...like for instance if my husband or another family member doesn't get them before 3:05 they are calling our house. Which half the time they're sitting down the street waiting for the cars to move because you can't get down the street. The dirty looks now and the rude manner in which were spoken to, I've even had my sons teacher make an accusation that my son said my husband wanted to take my son to a place where the kill children???? My son denies he ever said that and my husband was completely shocked. I just don't know what to do I feel so helpless. Should I just pull them out of school?

Worried parent in Oregon