Asalam alaykum,
My sister goes to a catholic school where they require them to attend mass which is a Christian religious service if the parents don't opt them out of it. There they sing which is their way of worship and it obviously includes words of kufr. She only moves her mouth without actually saying anything because if she didn't move her mouth the teachers will tell her to. The scary thing from what I understand is if she feels the pressure and joins in, she would be joining in their worship and if she says words of kufr that would actually be kufr as it's not justified as her life is not at risk. I have already advised my parent regarding this but unfortunately, they are sometimes negligent when it comes to such matters and are afraid to make a fuss even though I told them that it's haram and in some situations can lead to kufr. The sad part is how the law of the country actually allows the parent to take out their child and I showed her this but still to no avail. I was hoping you send a message clarifying this to her as she might listen to a learned scholar more than myself. I will continue to try my best. I would appreciate it if you can respond to me as soon as possible. Jazakum Allahu khayran for your efforts and may Allah bless you all.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is no doubt that it is prohibited to partake in the religious rituals of the disbelievers, and there is no difference of opinion among Muslim scholars in this regard. Allah, The Almighty, Says (what means): {And [they are] those who do not witness falsehood, and when they pass near ill speech, they pass by with dignity.} [Quran 25:72] In his commentary on this verse, Mujaahid said that it refers to the disbelievers’ festivals. The same was stated by Ar-Rabee‘ ibn Anas, Al-Qaadhi Abu Ya‘la and Adh-Dhahhaak . Ibn Seereen said: “In the verse, ‘falsehood’ refers to Palm Sunday, which is a Christian festival celebrated on the Sunday before Easter. Christians celebrate this festival by carrying palm leaves. They claim that this is the anniversary of Christ's entry to Jerusalem, as stated in Iqtidhaa’ As-Siraat Al-Mustaqeem (1/537). Ibn ‘Atiyyah said in Al-Muharrar Al-Wajeez: ‘The meaning of (the verb) ‘witness’ in this context is clear. It means they watch and attend, and falsehood here refers to all manifestations of untruth and adorned falsities.’” [End of Quote]

Since the school in reference prevents the student from attending such rituals and ceremonies (that involve disbelief) at the parents’ request, it is incumbent on your parents to fear Allah, The Almighty, and inform the school administration that your sister is not allowed to attend such rituals. Otherwise, they would be sinful and at great risk. It is impermissible for your sister to attend such ceremonies or utter the words of disbelief to conform with the disbelievers.

Allah Knows best.