one day at central high school , 1/12 of the students were absent, and 1/5 of those present were on a field trip. If the number of students

I can't see how the 1/5 is relevant  

1/12 were absent and 11/12 were at school

so 

$$\\\frac{11}{12}X=704\\\\
X=704\times\frac{12}{11}$$

$${\mathtt{704}}{\mathtt{\,\times\,}}\left({\frac{{\mathtt{12}}}{{\mathtt{11}}}}\right) = {\mathtt{768}}$$

there are 768 students in the school.

NOTE: Alan has interpreted it differently from me.  

Let N = number enrolled

N- N/12 present or 11N/12 present

11N/(12*5) on field trip, so 4*11*N/(12*5) in school or 11N/15 in school

11N/15 = 704

N = 15*704/11

N = 960

.