Directory Merge: unwanted condensing of key data

[MyerSL]

I am merging a directory of admissions decisions made on students from various high schools, following MacroPod's wonderful tutorial. I have everything working as desired, EXCEPT for random records it is condensing all into one.

For example, schools A, B, C, and D all have their own page with the correct letter template (attached) and merged records. Then, out of the blue schools E and F are condensed (one letter listing students from BOTH schools--- merging in the address from only E and then throwing the students from F in at the bottom). It then gives G, H, and I their own separate pages but combines J, K, L, M, N.

The data source quite clearly has the data changing for the key record in the schools where it's merging them (unwanted) so I'm not sure where I've gone wrong. The data source is alphabetized by the school address (to avoid errors thinking 2 schools with the same name are the same school) and then by student last name for ease of use. Do I need to remove the 2nd level of sorting and ONLY sort by school address?