Headteacher Debbie Rogan said she was 'mystified' as to why the parents did not attend the sessions at Briscoe Primary School, in Pitsea, Basildon, Essex, and she was 'disappointed' the children would not be able to start school with the other pupils.
As many as nine children, some aged just four or five, were banned for a week from after their parents missed the sessions. The children will be allowed back to school next Tuesday if their parents attend specially arranged meetings.
The 200-pupil school, which became an academy last September, holds four sessions for parents of children starting at the nursery and reception school.
Two of them, covering health and safety, child protection and the curriculum, are compulsory.
Parents were warned 'in writing, by text, on the website and in person, more than once' that their children would not be allowed to start at the school or nursery this week if they had not attended both.
Mrs Rogan said: "I find it very startling and I am mystified by the fact that parents have made the choice not to attend these sessions.
"My biggest sympathy is with the children and we are disappointed that they have not been able to start along with the other pupils."
But parents were left seething at the action.
One mother, Kaily Barnard, 38, said her son was turned away from the gates after she took him to the school yesterday.
She said: "I got my son dressed and took him to the school, I didn't think they would turn him away at the gate while he was in his uniform, but a teacher said his name was on a list of children who were not allowed in.
"I missed one session but it's not like I would have come away with a BSc in pre-school science.
"My son is not traumastised but some of the children were.
"There are decent parents at that school and the head is trying to make an example of us - she can do that if she wants to, but not at the expense of my four-year-old son."
She added: "I only attended one of the sessions for no other reason than the second one I just completely forgot about. But they never wrote to us and told us our child's start could be delayed."
The Daily Mail reported that one mother-of-four had lost her job because she had to tell her employer she had to look after her children for the week.
Mrs Rogan said the school follows the policy set by the HEARTS Academy Trust which states parents must attend two compulsory sessions with school staff before children start at its nursery or infants.
She said the sessions cover vital topics such as the health and safety of their children and the school building, the curriculum, safeguarding, the complaints procedure, attendance and their academic progress.
She said parents who couldn't make the first date had been invited to attend on a second date and that those who failed to attend on that date were warned on 'several occasions' that their children would not be allowed to attend their first day of school.
A HEARTS Academy Trust spokesman said: "The school and trust policy is that all parents attend two induction sessions with their children prior to pupils starting school.
"Starting school is an important step in a child's life and parent feedback is that they value those sessions very much.
"This year is the first year ever a group of parents who were informed in writing, by text, on the website and in person, more than once, chose not to attend despite a second session being run especially for them.
"We are as mystified as everyone else as to why this would be."