More than a dozen children hospitalised after eating an unknown substance on a north Queensland school bus have been released after a night of medical care.

Police and the education department are investigating how 22 students, aged between eight and 12 consumed the substance, reportedly in lollies, before feeling unwell at the Bluewater State School in Townsville on Thursday.

Paramedics said the children were treated for symptoms including nausea, abdominal pain and lightheadedness.

All 15 Bluewater State School children being treated by the Townsville Hospital and Health Service were discharged from care on Friday, the hospital said in a statement.

Bluewater State School said it could not provide further details due to privacy issues.

Queensland Police and the Department of Education have been contacted for comment on the investigation.

– AAP