Whether the person is drug tested as a condition of employment is dependent on the terms of their employment contract. That contract is between the employee and the employer. Generally, only parties to a contract may enforce its terms under the doctrine of privity.
A caveat to that general rule is that "third-party beneficiaries," though not in privity of contract, may enforce the terms of a contract. The parties to the contract must have intended to create the class of beneficiaries (usually there will be a clause in the contract expressly creating such a class of persons).
Assuming you are not a third-party beneficiary, you may lodge a complaint with the school board, or voice your concerns to school officials. Otherwise, you cannot compel the drug-testing of a school employee.
You want to be very careful, measured, and very discreet in voicing your concerns in this circumstance. If it is equally possible that this school employee has some sort of communications disability or disorder, consider couching your concern in some manner other than an accusation of suspected unlawful drug use. Also consider the nature of her employment and her sphere of responsibility for student welfare. This is a situation that should be evaluated and acted upon only based on careful consideration of the specifics of what is actually known and reasonably determinable.
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California public schools generally require pre-employment drug tests. If your child is in a public school, look at job postings on the district's website to see if drug tests are required. If it is a private school, policies vary.