Most will be in the second or third school year. The placement can be from 2 months to one full school year. This kind of internship gives you a chance to put things learnt in school into practice. This way the student gains work experience in their field of study. The gained experience will be helpful to finish the final year of study and provide an advantage in the labor market.
Sources:
http://www.prospects.ac.uk/work_experience.htm