Did you know that your brain develops until the age of 25? Anything that you do to disrupt this process—including substance—will affect how your brain develops.
During the brain’s development stage, any type of trauma and/or changes in the brain’s wiring could affect brain function. Drug use is one of the ways that can mess up the wiring. How?
According to NIDA, the brain relies on chemicals called neurotransmitters to get messages from one part of the brain to the other. Each neurotransmitter attaches to its own kind of receptor—like how a key fits into a lock. This allows messages to travel through the brain on the right path. When you use drugs, it interferes with the normal traffic patterns that the neurotransmitters use. The chemical structure in the drugs can imitate and fool the receptors, lock on to them and alter the activity of the nerve cells. This “alteration” can result in messages going in the wrong direction, and reset the way your brain should act or react.
Ultimately this affects the way your brain processes and retains information—and how you think, learn, remember, focus, and concentrate.
Research shows that there is a definite link between teen substance abuse and how well you do in school. Teens who abuse drugs have lower grades, a higher rate of absence from school and other activities, and an increased potential for dropping out of school.
Although we all know or hear stories about people who use drugs and still get great grades, this is not typical. Most people who use drugs regularly don’t consistently do well in school. 
Studies show that marijuana, for example, affects your attention, memory, and ability to learn. Its effects can last for days or weeks after the drug wears off. So, if you are smoking marijuana daily, you are not functioning at your best.
Students who smoke marijuana tend to get lower grades and are more likely to drop out of high school. One recent marijuana study showed that heavy marijuana use in your teen years and continued into adulthood can reduce your IQ up to as much as 8 points.
High school dropout rates have also risen as a result of substance abuse.
A study of teens in 12th grade (16-18 years of age) who dropped out of school before graduation are more likely than their peers to be users of cigarettes, alcohol, marijuana and other illicit drugs.
Illicit drug use among dropouts was higher than for those in school (31.4 percent vs. 18.2 percent). Dropouts were more likely to be current marijuana users than those in school (27.3 percent vs. 15.3 percent) and non-medical users of prescription drugs (9.5 percent vs. 5.1 percent).
Teens who smoke, drink alcohol, binge drink or use marijuana or other drugs are more likely than non-users to drop out of school and less likely than non-users to graduate from high school, attend college or obtain a college degree. One study found that nearly one-third of school dropouts indicate that their use of alcohol or other drugs was an important contributor in their decision to leave school.
Sources:
Adolescent Substance Abuse: America’s #1 Health Problem, The National Center on Addiction and Substance Abuse at Columbia University, June 2011
America’s Dropout Crisis: The Unrecognized Connection to Adolescent Substance Abuse, DuPont et al., Institute for Behavior and Health, Inc., March 2013
National Institute on Drug Abuse (NIDA) and NIDA for Teens
Substance Use Among 12th Grade Aged Youths by Dropout Status, The NSDUH Report, Substance Abuse and Mental Health Administration, February 12, 2013