Answer: 429
Explanation:
If all but 1/13 of the students were involved in the evening's activities, that means that 12/13 of the students were involved.
To calculate the total number of students, first you do a simple rule of three to know the number of students that weren't involved in the activities:
If 12/13 of the students represent 396 students, then how many students are 1/13 of the students?

Then, you add this number to the number of students that were involved in the activities:

So, 429 is the number of students that attend the school.
You can also calculate it directly by doing the following rule of three:
If 12/13 of students represent 396 students, then how many students are 13/13 of the students?
