The Utrecht Summer School office closes after the pick-up time. If you're scheduled to arrive in Utrecht after the pick-up time, you will have to pick up the key the next day. We therefore strongly advise our students to arrive during pick-up time on the Saturday or Sunday before the course between 12:00 and 18:00. The office is closed after 18:00 and there is no possibility to pick up your key anymore if you arrive in Utrecht later.
You could try to see if there is a friend or a course mate who is willing to pick up the key for you. However, you will have to make sure that you can contact him or her once you're in Utrecht yourself, as the Summer School staff will not be able to assist you on this.
If you can't make it, you could also book a room at the Strowis hostel (http://www.strowis.nl/). This hostel is a one minute walk away from the Utrecht Summer School office, and offers affordable beds in dorms, and some private rooms.