My eldest attended a vertically mixed class when he was in Y3 and 4. This was logistical, as due to a blip, his year was mainly boys and had 34 pupils and the year above had 18 mainly girls. They mixed the two classes along broad friendship/ability/interest lines and reverted to age-grouping in Y5. It was positive for my son as he made new friends whom he would not have considered had they continued to be in the year above or below. His teachers were experienced and competent and had not the least difficulty in setting appropriate work at the right level - "the younger ones want to keep up, and the older ones are determined they won't" was one comment - it led to competition of a positive kind. If your daughter and her friends are reacting to this minor temporary rearrangement with crying and terror, then either they have been misled and misinformed and very badly prepared, or been encouraged by someone to think this is an appropriate response, or this school is actually an institution for girls with emotional disorders.