In the article "Marijuana and Teens" it talks about how teen marijuana use is at its highest in 30 years. . In 2011, a national study showed that one in eight 8th graders, one in four 10th graders, and one in three 12th graders have used marijuana.
How to suspect when a teen is using Marijuana:

• Dizziness
• Acting silly for no reason
• Being hungry and eating more than usual
• Red eyes or use of eye drops
• Increased irritability or grumpiness
• Reduced motivation and lack of interest in usual activities
• Trouble remembering things that just happened
• A smell on clothes, or the use of incense or other deodorizers
• Owning clothing, posters, or jewelry encouraging drug use
• Having pipes or rolling papers
• Stealing money or having money that cannot be accounted for 

http://www.aacap.org/cs/root/facts_for_families/marijuana_and_teens