You are allowed to enter the exam room up to one hour after the start of the exam (which is why you can't leave in the first hour and the paper can't be given to your teacher until then).
However, if there are extenuating circumstances e.g. a major train derailment stopping students getting to school on time, then the BOS can direct that school's supervisers to allow the students the full time. Normally in that situation they get a misadventure appeal approved.
Being late due to oversleeping, not setting the alarm etc doesn't entitle you to any time or to misadventure.