Performance evaluations provide both the supervisor and the student with the opportunity to discuss concerns relevant to the assigned work and working conditions. On the most basic level, performance evaluations focus on performance problems and workable solutions. Performance evaluations also open the lines of communication between a supervisor and an employee, work to eliminate the “surprise” factor of future concerns or changes and promote the discussion of long-term plans and expectations. Additionally, performance evaluations prepare the student for the reality of the working world outside Winona State University. It is suggested that student performance be evaluated on a regular basis. A sample evaluation form (PDF) is provided.
You may want to consider hiring a new employee for a probationary period (whatever the supervisor feels is sufficient time for the employee to learn the job and demonstrate that s/he can meet expectations). Let the employee know that s/he will be reviewed after this period.
Keep the following in mind when giving a performance evaluation:

Be positive. Start on a positive note. What is the employee doing well? Tell the student what you like about their work, then explain the area that needs improvement.

Be constructive and specific. What are the actual examples of poor performance? What should be done in the future to improve?

Maintain privacy. Criticism should never be made within earshot of others.

Accountability is important. Students usually know when they need criticism. If you fail to react to unacceptable work, you may lose the respect of the student involved as well as that of other workers in the department. 
Be sensitive. Differences in abilities and aptitudes should be taken into consideration for each individual evaluation. The long-term goal of student evaluation is to assist the student in their development and to enhance the dimensions of the work experience.