Lindisfarne College in Hastings has expelled nine senior students for smoking cannabis outside of school hours.
The college is an integrated Year 7 to Year 13 school.
School Rector Grant Lander says another six year 12 and 13 boarders and day pupils have been suspended for between five and 10 weeks.
He says school staff will support them with work.
Mr Lander says when the students return to school they will be subjected to random drug tests over the next 18 months and required to have drug counselling.
He says it is extremely disappointing for the boarding school, which operates a policy of trust with its students.