Sitting highers/ advanced highers as an external candidate

My school won’t let me do extra exams. I know I am capable of doing so. Where can I sit these? I am aware I have to pay extra to do so. Does anyone know a place preferably in the Glasgow area?
Many thanks

Some colleges offer highers as evening courses, which might be an option. Alternatively, Wallace College in Edinburgh might be an option: http://wallacecollege.com/

Keep in mind that the extra qualifications (over & above what's needed/expected) won't make a lot of difference to things like uni applications...AAABBBB for example won't meet a AAAAA entry requirement. There's also lots of ways of learning without taking extra SQA qualifications too