CHICAGO -- Well, it's that time of year again. Time to stash a dead fish somewhere to stink up the school hallways. Time to drop tennis balls on the heads of people in the lobby. Time to cover your soon-to-be alma mater with Post-It notes.

For high school seniors, it's prank time – or "structured mayhem" in the words of Mindy Utay, a therapist who works with teens.

It's a rite of passage as graduation looms, mostly harmless fun but sometimes a escalating into vandalism. This spring alone, windows at school have been smashed, walls and sidewalks spray painted, and paint poured down steps. Cars have been flipped. Property has been damaged from California to Kentucky to Maryland.

As a result, school administrators are rethinking exactly what constitutes a prank and where to draw the line – and finding that's not always easy to do.

This year, the rule at Kenowa Hills High School in Walker, Mich., was clear: No senior pranks allowed.

But organizing themselves on Facebook, a group of graduating seniors there decided to ride bicycles, en masse, on the last day of school. They arranged for a police escort along the 3-mile route. The mayor even brought them doughnuts before they headed out to what was supposed to be a funny surprise for everyone else at school.

The principal was not amused.

She thought the students had put themselves in danger by riding along a busy thoroughfare. Traffic was disrupted. Drivers caught up in it, including some teachers, were late for work. In the principal's mind, the seniors had broken the "no pranks" rule, and she came down hard.

"But we didn't really see it as a prank. We saw it more as a senior send-off," says Sarah Pechumer, one of the 65 graduating students who participated. "It was harmless. It was arranged. It was legal."

And in the rowdy history of senior pranks, it was relatively benign. Recall the letter sent to parents at California's San Dieguito Academy in 2006, informing them that henceforth condoms would be distributed to students at all dances. Or the night at New York's Nyack High School, when seniors – with the blessing of their principal – arranged 1,000 school desks on a field to spell out "2008." Then, under cover of darkness, other pranksters (perhaps from the Class of 2009) re-arranged the desks in the shape of a giant penis.

Former students at one high school outside Hartford, Conn., still recall how their principal inadvertently sent them into hysterics after some seniors removed the plastic balls from the computer mice in a school lab.

The principal got on the intercom and began a lengthy speech about needing the "mouse balls" back – a result that even the students hadn't anticipated.

After a secretary interrupted him, he stammered and continued by asking for "the apparatus necessary for the computer mice."

"As long as it doesn't get out of control, I think it is healthy," says Utay, a therapist and clinical social worker in private practice in Manhattan. "It's something they look forward to after all the pressure – a chance to take back some of the control. It's rebellion against that pressure, empowerment. It marks the end of the high school experience."

Utay says, at her own children's school, pranks are considered a tradition – even a show of school spirit. Generally, it's been silly things, she says – dropping the tennis balls or hiding the dead fish.

Or, say, putting four pygmy goats on an overhang above a school entrance, as authorities at Simsbury High School in Connecticut discovered Wednesday.

"Like a lot of the practical joking and horseplay that goes on between adolescents and young adults, pranks are by and large fairly harmless, if nothing gets bruised except dignity," says Sherry Hamby, a psychology professor at Sewanee, The University of the South, in Tennessee.

Pranks often continue into college life, as they have for Brigham Young University junior Nate Stebbing and some of his buddies. This spring, Stebbing and his crew turned neighbors' apartment living room into a giant Easter basket, complete with real sod and live bunnies and chicks. The video they made has gotten hundreds of thousands of hits on YouTube.

The key to a successful prank, Stebbing says, is to never be mean-spirited or destructive. (The sod is now adorning someone's lawn and the animals found a good home.)

"We made sure that the people we were pranking were not people who'd take it the wrong way," he says. "Now we're super-tight friends with them – and it's an awesome memory."

When there is damage done, however, school administrators say it's important to set a firm limit – a precedent aimed at fending off future destructive pranks.

"The punishment should fit the crime," says Patti Caplan, a spokeswoman for Howard County Public Schools in Maryland.

Recently, students at one of the county's high school spray painted exterior walls and sidewalks at the school, threw toilet paper around, and moved trash cans into the middle of the street to block traffic.

Parents, Caplan says, didn't seem that concerned, as if "this is to be expected – that this is what you do when you're a senior."

But while she says it wasn't the worst prank they've seen over the years, school officials took it seriously. The 30 or so seniors involved came forward after they were told that no criminal charges would be filed if they confessed. They then spent the day before their recent graduation cleaning up the messes they'd made.

Even when there is no damage, some administrators feel the need to take a hard line. In May, in Clayton, Ind., a half-dozen students were suspended when they decorated their high school with more than 10,000 Post-It notes, and about 50 students who protested by cutting class were suspended as well.

Meanwhile, back at Kenowa Hills, principal Katie Pennington banned the students who participated in the bike ride from their "senior walk," a yearly ritual when seniors take the last hour of school to say goodbye to underclassmen and school staff.

After parents and students complained, Pennington issued an apology "for a reaction that blew this incident out of proportion and called into question the character of our students."

"My actions and emotion overshadowed what should have been a very positive senior activity," she said in her statement.

In response, one local radio station gave the principal a bicycle.

At their graduation rehearsal, seniors also wore T-shirts that said "Freedom Riders."

It was all in good fun, and senior Cara Dirkmaat said she learned something from the experience.

"We all make mistakes," she says.

Even adults.

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