Yes, health insurance is mandatory for all international students in the Netherlands. For stays shorter than 12 months where you are not working, you will need private insurance that fits what the Netherlands requires. If you stay longer, or if you get a job or paid internship, you might have to sign up for Dutch public health insurance (Zorgverzekeringslijn). You will need to show proof of insurance when you apply for your visa, and it needs to be valid for your entire stay. Some schools offer group insurance plans for international students.