This is a solution to Reverend Kirkman's schoolgirls problem in
combinatorics.
(See that writeup for more details.) If the 15 schoolgirls are denoted
by the letters a-o then a solution is given by:

Day 1  Day 2  Day 3  Day 4  Day 5  Day 6  Day 7

 abc    ade    afg    ahi    ajk    alm    ano
 dhl    bik    bhj    beg    cdf    bef    bdg
 ejn    cmo    cln    cmn    blo    cij    chk
 fio    fhn    dim    djo    ehm    dkn    eil
 gkm    gjl    eko    fkl    gin    gho    fjm