The Scottish Qualifications Authority administers the examinations you will sit in the Senior Phase (S4-6). They set the exam papers, mark them, and send out your results.
National 5, Higher and Advanced Higher courses are partly assessed by external examinations. The timetable for these exams is available online and the draft version copied below:
SQA also provides the helpful items below:
Looking for Past Papers to study from (with marking instructions)?
Wanting your results texted or emailed to you? My SQA
Looking to create a Personalised Timetable / Study Guide?
Want to know what the tricky areas are or how last year's exam went? Search for SQA course report and the subject and level.