The information's given in the question is sufficient enough for getting to the solution of the question.So it is very important to look at the information's given very minutely.
Number of members in the high school band = 250
Let us assume the total number of students in the high school = x
Then
[(3/15) * x] + 12 = 250
(x/5) + 12 = 250
x/5 = 250 - 12
x/5 = 238
x = 238 * 5
= 1190
So the total number of students in the high school is 1190. i hope the procedure is simple enough for you to understand. I am glad to have helped you.