You may disagree, but I see that as losing out. If for an hour a day, 5 days a week, one child is put in the corner of the room to sit alone, while the teacher gives their attention to the other children. If one child is isolated, and is not taught anything for 5 hours a week, while other children are? What about entire mornings when they go to mass, or entire afternoons dedicated to HC rehearsals?
And what about Easter and Christmas, and other catholic holidays? How many hours will be spent on nativitys and religious songs and prayers and even artwork of religious iconography? How do we opt out of all of that?
Perhaps if you were non-catholic you may not be that bothered, and may join in to a limited extent. But we are non-christian, staunchly atheistic, and we are faced with being in a position of having religious instruction practically forced on our children, because you can only opt out to a very limited extent.
Let me reiterate, I have no real objection to religious teaching in schools (in an ideal world I would prefer not, but practically speaking), and I appreciate that the majority is either actually catholic or happy to go along with catholic teaching. What I cannot understand is that in this day and age there is no option available to a non-catholic family to have their beliefs and needs respected in terms of equal access to education.