Tell your employer you cannot work from now until end-November
because you need to study for O level. He will definitely
understand and let you go. If he wants evidence, just show him (a
photocopy of) your entry proof.
If you still need/want that
job, tell the employer to call/SMS you (on your mobile phone) after
16 November, so that you
can forget about it now (and focus on O levels) and can continue to
work after O levels. Most likely the employer will want to hire you
again only at around the start of December, as it may be easier to
do the accounts.