I don't know but I've never heard of psychiatric medications causing a false positive for pcp but there can be other reasons for false positives with drug tests. Ask his doctor and discuss all the medications he takes and ask if they are any other reasons for false positives. Pcp use is rare. Its an extreme and dangerous drug that causes extreme personality changes and psychosis. On the other hand if it did turn out he was using it its good you found out as he would require rehabilitation. Speak with him and his psychiatrist together and see if you can find out what's going on.
You need to find out if the school uses the same kind of testing equipment as the CJT etc. Also next time he has a drug screen at school take him immediately for a second drug screen privately. That way you have 2 results to compare.
Also discuss this with his Dr as ILAD has advised.
Are the school drug screens random, without warning but the criminal justice ones pre-arranged?
As awful as it is to think it, if the other tests are pre-arranged he could be abstaining prior to those tests which of course he can't do with on the spot checks.