Guidance on which materials are permitted in an in-person exam is available on the Exams website https://www.st-andrews.ac.uk/exams/in-person/ . Permission to use a dictionary is decided by the Examinations Officer for your School. 

The Library is unable to reserve dictionaries in advance and is limited by the number of copies and demand. This means that we can't guarantee that a dictionary will be available to you on the day of your exam and we therefore recommend discussing the availability of dictionaries with your school including if you are considering purchasing your own dictionary.

Students are able to request to borrow a dictionary for exam purposes only. If the exam is in a morning this can be done the afternoon before. If the exam is in an afternoon then the dictionary should be checked out the morning of the exam. Dictionaries must be returned immediately after the exam