The kids not in religious ed took 'alternativa' - which consisted of virtually nothing in my kids' school. It was basically study hall. The kids were told to do homework, or sometimes they were just left to goof around. Hopefully things have changed and that time is put to good use nowadays.
I would never stick my kid in a Catholic education class if it "felt wrong". But that's me...
My husband and I only have a fetus for now, but when time comes around to make the choice about putting him into religion or not, my husband and I have already said we will not be choosing religious education in a public school. Both of us believe that religion as it is taught here has no place in the public school system, DESPITE the fact that every religion teacher we have worked with has been the best and kindest teacher in the school. (This is why I have a hard time being so vocally against religion in schools.)
(Disclaimer: DH and I are both teachers and have both worked in the public system. DH is a very strange Catholic: confirmed and adamant about being married and baptizing our son into the church, but VERY burnt out after his years in Catholic school. I am a UCC protestant.)
Depending on the teacher, religion can mean coloring pictures of Jesus or it can be one of the most difficult classes that one has. I taught in a Catholic school one year and would find my kids stealthily studying for Religion during my classes. I had to bite my tongue from saying "Is it really that hard!?" because I knew the nuns made it tough on them.
Ok, so a few important things about "reli" in Spanish schools:
1. Depending on the region,
plenty of kids choose not to take religion.
2. The new law governing the Spanish education system, LOMCE, has changed the importance of religion in a child's academic record. YOUR RELIGION GRADE IS NOW CALCULATED AS PART OF YOUR GPA.
3. "Alternativa" no longer exists. While what Kalohi said was very true ("Alternativa" was goof off time), the class is now called "Social and Civic Values" in Primary and "Ethics" in Secondary.
If you speak Spanish, take some time to read:
¿Catequesis en la escuela pública? La polémica está servida con el currículo de Religión - 20minutos.es
In summary, although I adore my colleagues who teach religion, I will NOT be putting my son into religion class if it still exists when he reaches school age. If the class was more like World Religions that I took in high school I would be all for it. Unfortunately, it's just an intense form of Sunday School that counts for a kid's GPA and takes up an academic hour that could be dedicated to something else, like arts education (which is being cut).
I'm sorry. I'm passionate about this issue. Please read the article from 20minutos