violation of probation

i was accused of harrasment via phone (it wasnt me) and was given a first time offenders program to do. I just had to do 20 hours of community service, write an essay and attend a class. I did all that. Well the week before i had to go for the last time to see the probation officer I made a big mistake and went to school while under the influence of drugs (xanax). I was caught and sent to detention for a week and was luckily not written a ticket. Im pretty sure he (po)doesnt know about it and pretty sure i will pass the drug test. What should i do. And i f he does find out about it what can i expect to happen.