changing number of teachers to number of full-time teachers

as requested by Angelique, because that matches the data we collect closer, see #163 .
@schafik @zmyao88 Could you confirm that for 113 as well, we pull full-time teachers, and not just all teachers? Starting 661, we have only asked about full-time teachers, but for 113, we have data for non-full-time that we should not use.