Size: From 30 kids in one room to 1,000+ across multiple buildings. Overwhelming.
Structure: From one teacher knowing you well to six+ teachers, none of whom know you.
Independence: Suddenly they're managing multiple classes, homework across six subjects, locker combinations, changing classrooms every 40 mins.
Social: Mixing with kids from five different primary schools. New social hierarchies. More complex friendships.
Academic: More independent work expected. More homework. Less hand-holding.
This is legitimately hard. Expecting a smooth transition is unrealistic. Expecting some bumps is realistic.