Some thing that you might want to look at are the number of free cards you are giving away. With many states having mandatory class size reduction class sizes are smaller and 15 would cover a lot of classes. My wife's class this year has been as small as 12 students and as large as 18 students. With 15 free cards she would just make the other three by hand. A lot of that is due to us already spending a large amount of money on her class due to budget deficiencies and salary freezes for all teachers for three years now.

Another issue you might be encountering is that states are adopting the common core standards http://www.corestandards.org/ and word bingo doesn't meet any of the standards for english language arts. Since everything in classrooms is documented, lesson plans are stored and in some cases handed over to the state there's not going to be a lot of use cases for activities that can't be applied to the standards. Even if those activities are just for fun due to a holiday.