If the school actually offers the course or level then they cannot stop you doing it unless the they need to set up a criteria and the class is full e.g. to do Advanced you need an A or B in the SC and we will only be able to run one class of 25 and 25 meet that criteria (25 is the maximum number in State high schools I think for a Senior class but I may be wrong on that number).
If the class isn't full then you can insist on doing it and contacting your school's BOSLO officer will find that that is the case.