I don't think you should be thinking of it in terms of "pushing their agendas." It's not like they're going door to door trying to convince you to convert to their way of thinking. You voluntarily sent your kids there and you PAY them to teach your children this stuff. They aren't trying to convert anyone, they think they already have a captive audience filled with children whose families believe the same things they do. I feel your pain regarding having an awful public school, but I think in that case I'd either move or send my kids to a non-religious private school. The philosophy of a religious school is the hope that the parents want Catholicism ingrained in their children, and the values and teachings of it interwoven in every subject. In my experience, it's the families who send their kids there looking for superior educations and can't agree or live with the doctrine that are the least happy.
That being said, I went to Catholic School and my kids (3 sons) go there now. My oldest is graduating from 8th grade in 2 weeks and has been going there since Pre-K. I doubt homosexuality was ever discussed in any grade. Perhaps they may have said that the Church feels it is wrong, as well as premarital sex being wrong, but my kids have never been threatened with the fires of Hell with anything. As a matter of fact, all the teachers, including the Principal are lay. There are no nuns to be found. And I've never seen our Priest regularly even step foot in our school building. The 8th grade homeroom teacher was reading the DaVinci Code on the bus on the way to our NYC field trip. I"m sure she was just keeping up on current events and not "believing" what she was reading. What Ally&Jack went through in 1990 is blatant child abuse...I can't believe that was tolerated. Even when I graduated from 8th grade in 1981 and had several nuns and the priest had an office inside our school back then, I remember no "mortal sin/you're going to Hell" speeches. The closest I've ever come to that was a graphic Pro-Life film that was shown to us in my strict, All Girls, Catholic High School.
I feel bad for how hard this must be for you. We are moving this summer and my kids will attend Public School. I'm sure that will be a total culture shock for all of us.