Transitions are difficult. Your brain prefers consistency.
Now imagine a typical secondary school. Tutor time, lesson 1, lesson 2, break time, lesson 3, lunch time, lesson 4, lesson 5.
Each of those activities take place in a different location, with a different set of peers and a different teacher in charge.
Each day, the timetable is different, for either a week or a fortnight.
Every so often, the location or teacher changes with no advance notice. Or the whole day is different - sports day or a trip somewhere.
You find it really difficult but everyone around you is fine, you don't understand why. It's really tiring every day and you can't wait to get home. But when you do, there's school work to do that didn't fit into the school day. You don't want to do work at home, home is your relaxing space. You get in trouble for not doing work.
That's without even factoring in sensory difficulties, social and communication difficulties, executive functioning difficulties.