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.