Impact Evaluation of Anxiety in the Classroom Program

I am hoping for guidance on impact evaluation of our Anxiety in the Classroom program - www.anxietyintheclassroom.org . This is a freely-available resource with a lot of variables by design, as our intent was for it to be usable by anyone (e.g. students, caregivers, school personnel of any profession, mental health professionals, etc.) anywhere in the US. We are hoping to collect baseline and follow-up data to help demonstrate the impact of this program, but due to the complexity we are struggling to design our evaluation measures/processes. We have piloted this program and have several other implementations planned in the coming months, in addition to developing a "Train the Trainer" model for its ongoing dissemination. Any guidance on how to best set ourselves up for measurement success (and outcome/impact reporting thereby) would be much appreciated.