It was drugs.

RadarOnline.com was the first to report that Sean Penn's 16-year old son was arrested at school last week, and now, we can reveal why.

EXCLUSIVE: Sean Penn's Son Arrested At School

Two sources familiar with the situation exclusively tells RadarOnline.com that the teen was arrested for having narcotics in his possession.

As we first reported Tuesday, the younger Penn was busted at the school he attends in the posh Malibu area.

• TAGS

• arrested
• Drugs
• Hopper Penn
• Malibu
• Narcotics
• Robin Wright Penn
• Sean Penn