We have pupils in y2 who have scaled a 10 foot fence to abscond from the school.
This is a mainstream school. These pupils do not have statements.
If your child was in my class, would you be happy with me leaving the classroom everytime a child decided to walk out. Due to the challenging behaviour of some children in my class, this can happen up to 20 times a day on a bad day.
Again, this is a mainstream school and the children are 6 and 7.
I know my class is a fairly extreme case, but each school makes its decisions about child safety and welfare based on the children in each class. We have made a very definite decision that neither the teacher or TA will leave the classroom to follow an absconding child. We will check through the door to make sure that they are in the library next door, and then we go out to them at an appropriate moment.
Can you imagine what would be said on Mumsnet about a teacher who constantly left her class to focus on one child, to the detriment of others.
The issue at stake here is that the OP's little boy is unhappy about something. That defintely needs to be investigated. However, the OP has also said that there were difficulties at his last school too and so a certain amount of personal reflection needs to take place on what may be causing his behaviour.
If his current school has never had children before who have left the classroom before, then they would not be expecting to have to check constantly to make sure that all the children are there.
When we go out on school trips, we count the children constantly because it's an environment unknown to them, full of exciting distrations etc. However, in the classroom, I rely on the children in my care to know and understand the boundaries and expectations because we will have talked about them many times.
It is not unreasonable to expect a 5 yr old NT child to understand that it is wrong to leave a classroom or agreed area without telling someone. If a child is determined to leave, and hide, then they will find a way to do it.
Of course the OP needs to talk to the school about this, but the conversation should focus more on why the OP's dc is unhappy and choosing to opt out, than why the teacher didn't notice he was gone.