Utrecht Summer School offers basic student accommodation to its participants. If you prefer to book a hotel / hostel room it's your own responsibility to make a reservation.
You can find some alternative options here:
Hostel B&B Utrecht
Hostel Stay Okay Utrecht Centrum
You can also check out Airbnb and websites such as booking.com to find accommodation in Utrecht. If you do not book housing through the Utrecht Summer School it is your own responsibility to find housing.