Write and solve an equation of proportions:
1 adult 15 students
------------- = ------------------
x 276 students
Cross-multiplying, 276 = 15x. Dividing both sides by 15, we get x = 18.4.
Since it's difficult for 0.4 of a person to get around without help, the school will probably require 19 adults to chaperone these 276 students. ;)