"Last week a Grade 9 class teacher at a school on the South Peninsula noticed signs of substance abuse among pupils. As a result 15 pupils were tested for substance abuse at the end of July," Zille said yesterday.
"All 15 tested positive.
"But this number increased to more than 40 when pupils, who after realising they could also be tested, voluntarily admitted to using drugs."
This was the first mass testing to take place in a school in the province.
The outcome was that close to 50 percent of the grade, all around 14 years of age, were using drugs.
"It is clear from this case that we urgently need to step up intervention and treatment programmes that target young people," Zille said.
She said the provincial education department would send a letter to every principal in the province this week to provide guidelines on how to implement drug testing at schools.
"About 80 percent of crime in the Western Cape is linked to drugs," Zille said.
"The scale of this problem is enormous. What we have to do is capture children before they start experimenting and sink into chronic addiction."
Key to "turning off" the drug supply tap was for pupils to know they were going to be tested regularly.
"It has been found that if young people know they are going to be tested regularly, they will be less likely to start using drugs."
In many cases, she said, children were self-medicating on drugs to deal with a crisis in their lives.
Western Cape coastal communities were specifically at risk as drugs were closely linked to the illegal abalone trade.
Dealers had been found to be setting up tik factories in places where the abalone was being swapped for drugs.
Robert MacDonald, the province's spokesperson on drugs, said the police had been very successful in raiding drug dens across the province.
He said, however, that courts were packed with 30,000 drug cases, which often took long to resolve because they involved substances that had to be analysed.
Mental health facilities were also overburdened with patients who had been brought in for drug-induced illnesses.
The health department, as part of its 2020 healthcare plan, was looking to scale up mental health services.