It depends on what the letter says. Unless the letter specifies that certain notice must be given prior to not taking or leaving that position, or is in the form of a contract obligating you to teach for a set period, then yes, you can do this: in the absence of a written agreement which by its very terms limits your ability to leave or terminate employment, employment is employment at will, and you may end it (even before it begins!) any time you want, on no prior notice, and for any reason. Review the letter, therefore, to see what it says; if in doubt, bring it to an attorney to review with you.