But they should remember they are also dealing with children; young people who will experiment and make mistakes in a moral climate and with personal choices about which adults - including teachers - are also often ambivalent.
The distinction between carrying drugs in school and supplying needs to be recognised along with the disastrous consequences of exclusion from qualifications and employment opportunities which in turn increases the risk of falling into drug addiction and crime.
Most heads and governors appear to use permanent exclusion only in cases of repeated possession or dealing. But there are signs that some efforts to tackle drug abuse through school-based prevention programmes are being undermined by excessive use of the exclusion weapon.
As Tony Blair said in The TES in September: "Poor education, crime, truancy, drug addiction, homelessness and unemployment are all linked. We have to link up the solutions." In the fight against drugs, blanket exclusions are not the answer.