If your child is just starting on medication, explain what it's for. For example, let your child know, "It will help you calm down... it will help you pay attention." Emphasize that it's not a magic pill, however - he must also work hard to follow the rules and make good choices.
Work with your doctor to find an effective regimen. Your child should be on meds whenever symptoms interfere with her success in life. She may need coverage during school hours and beyond, to help her deal with homework, after-school activities, social relationships, even bedtime.
Keep tabs on how long the medication actually works. A "four-hour" pill may last three or five hours; 12-hour meds can last from 10 to 14. A combination of short- and long-acting stimulants can cover the full day.
Ask the teacher for feedback. Has she noticed a difference in behavior or focus? Is there a point in the day when the effect seems to wear off?