My coworker left me a nasty note...what should I do?

I received a note in my mailbox at work yesterday, which said I should quit my job...not surprisingly, it was anonymous. To make a long story short, I work at a child care center and am a certified teacher. I love what I do and do it to the best of my ability. There are several people I work with--including a supervisor--who are a clique, who hate their jobs and have tried, sometimes successfully, to force out teachers with education. They tend to gang up on one person at a time, and right now, I feel I'm that person. What should I do? My boss is unpredictable and is not good at resolving conflicts. This whole thing has been a nightmare. I did keep the note. Thank you for your time.