Sean's dad is the most skilled wizard in Manchester. He was valedictorian with an affinity for rapid casting, and got the alliance on the strength of that and got them all out, and married a Manchester girl, and has four children, though Mum had health problems for a while that mean Sean is eight years older than the next one. Everyone expects his kids to be amazing too.
Sean is not amazing. He has English and French, and his parents advise him to do Latin in language lab because they think the curriculum for it is better than for other languages they have information on and the course offerings and spell selection will be good, but they haven't risked trying to start him on things in advance since he isn't actually that quick at classes in anything else. He has an affinity for water magic, which will not be very useful in school and doesn't suggest a particular track for him, so he's probably going to have to study more than just Latin if Latin doesn't start trying to eat him alive. He's tall, so he's not carrying much - Poppy has some of his stuff - and he feels like a waste of space, but he doesn't exactly have the option to say "Dad, let's find an indie from Vietnam to send in my place", it would sound sarcastic if anything. So here he is. He pets his baby sister's hair. She won't remember he exists, if he makes it back at all.
He is already feeling so nauseated when the induction hits that it doesn't make a difference.