Key word is openly gay. That means they live the lifestyle aka engage in the behavior.
As others have mentioned, this is not what “openly gay” means. It refers to people whose attraction to people of the same sex is publicly known, regardless whether they engage in sexual acts with people of the same sex. Public acknowledgment of this attraction no more requires sexual expression than does public acknowledgment that one is attracted to people of the opposite sex. In the words of the USCCB, “Do not presume that all homosexual persons are sexually active.” There are lots of openly gay Catholics and other Christians who are celibate and, like many straight Catholics, strive to live chastely. Eve Tushnet is a Catholic example, Wes Hill a Protestant example. I have friends who are openly gay, faithfully Catholic, and celibate. They’re a great blessing in my life.
However, where is it written that a gay student is entitled to go to a Parochial School
Reading Church teaching suggests that basic human rights may only be abridged or restricted in cases where public conduct endangering the common good may require it. The CDF has written, for example:
"Congregation for the Doctrine of the Faith:
Homosexual persons, as human persons, have the same rights as all persons including the right of not being treated in a manner which offends their personal dignity. Among other rights, all persons have the right to work, to housing, etc… {These rights} can be legitimately limited for objectively disordered external conduct… it is accepted that the state may restrict the exercise of rights, for example, in the case of contagious or mentally ill persons, in order to protect the common good.
It does not appear that merely knowing a person’s sexual orientation would be adequate grounds for barring school attendance. Such an action would, I think, be reductionist in a way discouraged by the Church:
Congregation for the Doctrine of the Faith:
The human person, made in the image and likeness of God, can hardly be adequately described by a reductionist reference to his or her sexual orientation. Every one living on the face of the earth has personal problems and difficulties, but challenges to growth, strengths, talents and gifts as well. Today, the Church provides a badly needed context for the care of the human person when she refuses to consider the person as a “heterosexual” or a “homosexual” and insists that every person has a fundamental Identity: the creature of God, and by grace, his child and heir to eternal life.
I could certainly understand a school disciplining students for engaging in sexual activity, at least on school property or at school events. (I would expect the school to have a policy like that for all students, regardless of sexual orientation. It seems probable to me, anyway, that a school’s standards of moral conduct would apply to both heterosexual and homosexual students.)
Like it or not, gay male students are subject to harassment by straight male students, which can lead to other problems.
In a scenario such as this, it is the straight male students who should be corrected and disciplined. This seems the course of action most consistent with Church teaching:
Catechism of the Catholic Church:
The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God’s will in their lives and, if they are Christians, to unite to the sacrifice of the Lord’s Cross the difficulties they may encounter from their condition.
Congregation for the Doctrine of the Faith:
It is deplorable that homosexual persons have been and are the object of violent malice in speech or in action. Such treatment deserves condemnation from the church’s pastors wherever it occurs. It reveals a kind of disregard for others which endangers the most fundamental principles of a healthy society. The intrinsic dignity of each person must always be respected in word, in action and in law.
Congregation for the Doctrine of the Faith:
Moral conscience requires that, in every occasion, Christians give witness to the whole moral truth, which is contradicted both by approval of homosexual acts and unjust discrimination against homosexual persons.
U.S. Conference of Catholic Bishops:
We call on all Christians and citizens of good will to confront their own fears about homosexuality and to curb the humor and discrimination that offend homosexual persons. We understand that having a homosexual orientation brings with it enough anxiety, pain and issues related to self-acceptance without society bringing additional prejudicial treatment.