How can I address several deans in a letter? I am part of the student council at my college and I would like to email a letter to all of the deans.

How can I start the letter without writing "Dear Deans?"
One solution would be: instead of sending a single email addressed to all of them, send each one a separate message, addressed to them personally and with an appropriate salutation.  This is also more likely to get their attention than something that appears to be a mass-mailed form letter.  If it's important that they know the same letter is being sent to the other deans, say so in the text.