Make a detailed grading system. A work-for-profit is a job like any other, and the student can be held accountable for his or her effort and results. So create a list of ways a student can prove his worth performing work-for-profit. Filling in for others, being reliable and advancing are good ways to judge how well they are working.
Keep a schedule of the student's activity. Most classes consider attendance as part of the grade. So, be sure the attendance at the work-for-profit is also up to date.
Meet with the employer. Whether it is an adviser at the school, or a private business manager, or another teacher, meet and discuss how well the student is performing his tasks.
Weigh all of these grounds for grading as separate -- putting a little less leverage on attendance -- and average them together. This removes some of the subjectivity of the grading process.