What could they arrest me for?

they could take you home, if the even do anything, if she says you ran away or were kidnapped or somthing to make them do somthing she might be the one who gets in trouble. all in all she sounds like a nightmare and i dont blame you for rebelling, i doubt you could be arrested for sneaking away to a class trip since its not the same as sneaking out after curfew or whatever. good luck.