The school in question is a grammar school, typically meaning 1st through 5th grade. My memory is a bit fuzzy from my time as a grammar school student but I'm pretty sure that the dictionaries for use young children were highly abridged. The dictionary that is in question is a Merriam-Webster’s 10th edition. Was it for kids, for high school students, for college students? No word on that.
The point I'm trying to make is that it's not unreasonable to say, "A college level dictionary is inappropriate for grammar school students."
That said, I know I and many of my male friends looked for dirty words in the dictionary. It's what boys do. (Sorry ladies I can't speak for you on this one.) The definition given is pretty vague to be fair: "oral stimulation of the genital."
With the Internet widely available to kids, with or without supervision, this is not the worst thing in the world. Better that they are educated than misinformed, yes?
EDIT: This article on Salon.com puts it better than I could.