If you want to know which opportunities a volunteer signed up for all you need to do is search for their record on your volunteer list, and then click the LOG HOURS link on their record. The page will show each of the opportunities, at each school. In addition this will show the school generic opportunities for each school on the volunteer's record.
You can also see the total number of hours the volunteer has logged on their record for the current school year and total hours throughout their career.