You said "was dismissed" so we can assume that was in the past, also, if you were dismissed, possibly you are not on the best terms with the school. Ergo, you ask them for a letter stating your last day of employment, that is likely to be in the past, therefore you could already be on overstay.
ok , that and your work permit need cancelling. A letter from work stating your last day of employment, take to Labour, then take the letter from them to immigration.