A 15 year old is on probation and he had his first drug test but failed it because of tabs.So what are the conquenses for failing your first pee test while on probation?

The consequences possibly include revocation of probation.

Your probation can/will be revoked. You will have to attend a violation of probation VOP hearing. At that time the judge may order you to spend some time in jail or give you more probation on top of your already existing probation depending on why you are on probation in the first place. To put it mildly, if you are on probation for any kind of drug charge/use and you test dirty, don't expect the judge to go lightly on you at your VOP hearing as you obviously have not learned your lesson and are still thumbing your nose at authority. Judges are quick to note this in young offenders and act accordingly.

They may or may not revoke you on the first test, it is hard to say, but it is not a good way to meet and greet your new PO.

You risk being arrested and have a hearing and could go to jail for the entire probation time, instead of probation.