Do not lie about your next position. That will end up pissing them off more in the long run. Keep it basic and throw in something about what a wonderful opportunity working there has been, how amazing the students are, how you will miss them, etc.
Please take this as my letter of resignation, with my last day being XXXX. It has been an honor working for "Company" and I value the knowledge and experience that my time here has given me.
Thanks Me
They don't need to know why, just that it is happening.