Students who leave school in the spring and look for work are one source of

The correct answer is frictional unemployment.
This refers to the type of unemployment which occurs when people are either between jobs, or changing jobs often. When it comes to students, they want to find a job during spring holidays, so that they can earn some money. However, when school starts again in autumn, these students usually stop working, and thus become unemployed again. This is what causes friction in the employment system.