A PRIVATE Brisbane girls' school will conduct random breath tests of its students at a Year 11 semi-formal because of concerns about them drinking at parties beforehand.

Moreton Bay College has taken the stance for the first time after what its principal says were "four instances" of students being sent home from their formal because they had previously been drinking.

But principal Jennifer Haynes said those incidents were not the only reason for the introduction of breath tests.

Rather, she said it was part of an overall strategy the school had formulated over 18 months to educate students and parents.