By Maggie Taggart
Education correspondent, BBC Northern Ireland
Integrated education is 25 years old, but the BBC has learned that in more than a third of integrated primary schools, local priests will not enter the buildings.
Primary school is a crucial time for the religious education of Catholic children; their first Confession, First Holy Communion and Confirmation all happen in those years.
Integrated schools have Catholic and Protestant pupils
In Catholic primary schools, the local priest is usually a frequent visitor, but in some integrated schools, the only time children see him is in church.
We surveyed 35 integrated primary schools and found that in more than a third of them, priests turn down repeated invitations to events there, or to become chaplains.
Some schools do have good relationships with local priests but, in a number of cases, principals asked us not to mention this, in case the priest got into trouble with his bishop.
Sometimes nuns and clergy from neighbouring parishes visit to give religious instruction.
In the early days of integration, the church was unwilling to give the sacraments to children attending those schools.
Things have improved and children who want to can take the sacraments, but there is still a reluctance among some priests to set foot inside integrated schools for social events.
Some priests still refuse to enter integrated schools
Many schools told the BBC they have tried for years to encourage the priest in, but without success.
A number of priests visit but do not want to take assemblies, as ministers of other religions do.
Some principals felt it was such a sensitive issue, they were reluctant to be quoted and concerned that they might rock the boat and destroy years of careful nurturing of the relationship with the Catholic parish.
They said Catholic parents feel they get a second class service and feel discriminated against by their own church.
Hazelwood Integrated Primary School in north Belfast has the largest number of Catholic children - almost 300 - in any integrated primary.
Yet despite years of invitations, they say a priest has never visited the school.
The school's principal, Jill Houston, said: "I am disappointed that the priest does not come in - we would welcome him.
Hazelwood has a large number of Catholic children
"In a school like this one with hundreds of Catholic children and situated on the peace line of north Belfast, we should have stronger links with the local Catholic parishes."
There appear to be three main reasons for the refusal of some to become chaplains for the integrated sector or to even visit them.
There is a shortage of priests so they cannot cover all schools
There is a loyalty to the local Catholic school and priests place a lot of importance on children having a purely Catholic education
There is also a problem with how much control a priest would be permitted in an integrated school
Until September 2006, Fr Martin O'Hagan was a diocesan advisor on education for 13 years.
"If I was invited to be a chaplain," Fr O'Hagan said, "I would have to set down the parameters of what I would expect.
"I would look for considerable accommodation in terms of reaching the Catholic pupils and would like the freedom to do so."
Simply to be a visitor to the school is, he said, unsatisfactory.
There are, however, examples of Catholic clergy with strong links to the integrated schools and, in all cases, priests carry out communion and confirmation ceremonies for the children.
The integrated movement has also been charting links with clergy. The picture is varied.
Some schools do not invite any in, others have Catholic nuns or brothers from outside the parish to talk to the children.
Denominations other than Catholic are much more likely to send in chaplains.
The chief executive of the Northern Ireland Council for Integrated Education, Michael Wardlow, said he knows some priests have difficulties going into integrated schools because of theological issues.
He says: "Things are improving but they are not where I would want them to be.
"The thaw has begun but I would like it to be less icy."