roisina and yet plenty of schools do consider buses as part of their planning for exams. Mine certainly does. The entire mock timetable has been thought out so pupils with extra time won't miss their bus. We've been told that it's "unavoidable" for all three maths papers to be in the afternoon because they are only 1.5 hours long so even with extra time fit into the 2 hours available in the afternoon, whereas any subject that has 2 hour exams must be in the morning so the extra time doesn't make them miss their bus.
Even in the real exams school have been known to make special accommodations for pupils where extra time has meant they will miss the school bus. Sometimes that has been a member of staff taking the child home, other times we have managed to book a taxi (difficult, because all the local taxis are booked up every day taking kids home from school as a block booking already), very, very occasionally the child with the transport/extra time issue has started their exam early (as allowed by exam boards, everything is done entirely by the book!) Starting the exam early is not ideal due to the noise of lunch break still going on around the exam room, which is obviously very distracting.
I think how accommodating schools are prepared to be with transport issues is generally to do with how many pupils are affected. In our school, because we're in a pretty rural location, approx 75% of our pupils rely on school buses, so we bend over backwards to ensure the kids are on those buses as they may have no other way to get home (public transport is unavailable to many of the villages our pupils live in). Dd's school is far less accommodating of buses, because they are in a city and pupils either walk or come on public buses which are pretty frequent. So different situations lead to different responses from the schools, neither is wrong, just reacting to different circumstances.
OP, I really hope that when you explain the situation to the school they can give some good suggestions about how it can be solved. Unfortunately, the most likely solution will be to give your son somewhere to sit and revise quietly until you can arrive to collect him.