An official-looking prank letter telling students to attend a “mandatory vagina inspection” in the gym at Prior Lake High School has parents, students and teachers wondering who’s responsible.

The letters were written on imitation school letterhead and hand-delivered to some families’ mailboxes this week. It’s unclear how many were distributed, but the Postal Service removed about 50 of them from mailboxes when they were discovered, said Kristi Mussman, a school district spokeswoman.

An image of the letter was shared widely on Twitter, Reddit and other websites. And it was a “topic of conversation” at the high school, Mussman said.

“It’s definitely a hoax. We believe a student or students created it after school hours,” Mussman said.

After the school’s police liaison officer saw a tweet about it on Tuesday, Principal Dave Lund sent out an e-mail to parents explaining that administrators “are aware of this letter, and we are addressing the issue internally.”

Mussman said the prank was “done in extremely poor taste” and administrators were “disappointed.”

Determining who wrote the letter is a police matter. The liaison officer has “some strong leads,” she said.

Because there is no evidence that it was written at school or printed on school property, administrators aren’t sure whether they could punish the offender, should they discover the person’s identity, or what that punishment might be, she said.

“I’m sure there have been pranks [here] in the past,” Mussman said. “This is obviously one of the bigger ones.”

Putting anything — fliers, letters, advertisements — in residents’ mailboxes without paying postage is a federal crime and could be either a misdemeanor or a felony, said Jeff Long, spokesman for the U.S. Postal Inspection Service in Minneapolis.

But most likely, if a mail carrier caught someone distributing letters, the offender wouldn’t face charges. The items would be removed, and the person would be told the action was illegal and be asked to pay for postage.

Lund said that with only weeks of school left, staff members and students are trying not to dwell on the prank. “We are moving forward to finish our year strong,” he said. “We have a very good student body … and we are not going to let this prank diminish the positive performance of our students.”