What can I do if the HR Director told the Principal through another staff member that I smoked marijuana?

I was called into my principal's office at the charter school I worked at and was told by the human resource director via phone conference that another staff member had reported I told them I smoked marijuana. I said that was not true because I do not smoke marijuana. I then stated that these were serious accusations and I asked if I needed representation. The HR director asked me why I needed representation and if I was guilty of something. I said no but I had never been in a situation like this and I didn't know what to say. He then stated that if I was going to request representation the conversation would end and they would terminate me. He asked me to submit to a drug test and be placed on leave until the results came back. I said no. I stated that I had worked there for 5 years and never had anything like this happen before and I felt it was unfair to ask me to take a drug test when they had no evidence besides an employee that was giving false information. After I said that the principal spoke up and stated that I was an excellent employee and was always on time, professionally dressed, and an award winning teacher. The HR director said a test was mandatory or I would be terminated. I said I would take the test. After I agreed, HR asked me if I was sure I wanted to do that. He said if I tested positive my teaching license would be revoked and I would never teach again. I said yes of course I want to take the test. He then asked me again if I was sure because they hair test and that goes back years. I did not know the testing protocols and I admitted that I had smoked marijuana once over summer vacation 7 months ago. He then stated that they would be separating employment because they are a 0 tolerance company. I was given the option to resign and I did. I requested my personnel file shortly after. In my file was an email to the HR director from the HR director stating that I was called into the office because staff had reported I was under the influence.