I mistakenly assumed Pate's didn't mix up groups, as I knew they stayed in tutor groups from yr 7-11 and these were linked to their houses. For yr 7&8 they were taught in tutor groups (I seem to remember there was some broad setting in PE and Music for more able ones). But in year 9 they mix up the teaching groups too. We had a letter to say they were mixing them (they do this every year) to ensure there was a good balance of backgrounds and abilities in each class, and all core lessons (English, all sciences, core RE, IT) are taught in these classes. As they also chose their GCSE options in yr 8 they have their option classes too. My daughter was not particularly fazed by it. They were told parents would have no say in the swap and all she said was "they've split all the trouble makers up!"
Don't worry, I'm sure it will work out fine