You need to stay in touch with your child's school. Never assume that their school will contact you if there is a problem. Kids tend to take off during the middle of the school day and get stoned somewhere near the school's grounds. Don't assume that their school will let you know about this kind of behavior. You need to realize that kids are great at covering up.
Every kid knows how to forge their parent's signature - no joke.
Call your child's school from time to time and ask about your child's attendance record. Don't assume anything - find out for yourself. Lack of motivation in school. Your child should show a healthy interest in school. If your child doesn't, then something is wrong and drugs are one possibility.