Is it haram when I also stand up when my non-muslims pray?

I live in a non-muslim country and I study in a catholic school and every begining of class the teacher would make us stand up and pray. The thing is, I stand up too because it our teacher would say that we should all stand up. So I just recite a surah when they're praying and I say "astaghfirullah" everytime. My question is, is it haram that I also stand up when they're praying?