Adam Hamburg is winding down the most stressful month of the most nightmare-inducing, backpack-bursting year of his life: 11th grade.

May has brought a barrage of standardized tests - the SAT and three Advanced Placement exams - with three more SAT Subject Tests to come in June. Hamburg plays on the tennis team, tutors at an elementary school, and has lined up a summerful of activities that include learning Japanese and Portuguese, attending business camp, doing volunteer work, visiting prospective colleges, and preparing his college essay.

Before the year ends, he has one more task on his to-do list: decide what he wants to do with the rest of his life. The coursework he can handle. Figuring out his earthly purpose at age 17 is another matter.

"It's really tough," said Hamburg, who has a near-perfect 4.72 average at Pennsbury High School in Fairless Hills. "It's a lot of pressure."

To an academic high-flier, nothing compares to the stomach-churning realization that time is running out in the last year that really counts toward getting into college. No more do-overs. The grades he has are the ones he's stuck with.

"Everything you do, you think, 'This is going on your transcript. This is deciding your future. Don't screw it up,' " said Emily Irwin, 17, a junior at Penncrest High School in Media, who it taking two AP courses and coedits the school newspaper. It's much more serious than ninth or 10th grade, she said.

With 3.6 million high school juniors in the country, two-thirds of whom plan to attend college, students who aim for upper-tier schools are facing increased competition and tougher entry standards.

Even non-"reach" schools turned down top achievers in record numbers this year - a fact many juniors are painfully aware of. And the baby bulge means next year is likely to be worse.

"I have this fear that I'm going to apply to so many different colleges and not get into one of them," said Katie Gault, 17, ranked 11th in her class at Penncrest, where she also carries two AP classes and a boatload of extracurricular activities.

To get an edge, students will sacrifice their downtime even after the semester ends.

Kelly Saam wants to play lacrosse this summer. Her mother wants her to tutor inner-city children. "She thinks it will look good on my transcript," said Saam, 17, an athlete and award-winning French student at Harriton High School in Rosemont.

Something as trivial as a bad quiz grade can induce meltdowns in students who also are trying to squeeze in junior proms and learning to drive.

"It's from the exhaustion," said Irwin, who recently took two AP exams and the SAT, and still has SAT Subject Tests and her regular exams looming. "You just want to go to sleep."

In the last two weeks, David Zdancewicz has sat for five AP exams, which can lead to college credit, in calculus, biology, physics, English and microeconomics.

"You're at the point where you have to get everything done and together and wrapped up so you can put in on your application next fall," the 17-year-old Pennsbury student said. "It's overwhelming."

For Zdancewicz, that included a second shot at the SAT, on May 5. He'll take more SAT Subject Tests on June 2, and he's beefing up his transcript with community service this summer.

What has him sweating bullets, though, is deciding what to study in college.

"Figuring out what you want to do, what college to apply to for whatever your career is going to be - if you don't know, that's the biggest stress," he said.

His anxiety is shared by many 11th graders, who are caught in the grip of hormones, adolescence and emerging ambitions. One of her greatest fears, Gault said, "is going to college as an undeclared major."

Asking 17-year-olds to identify at least a general area of interest is difficult, but it helps them choose where to apply, said Jim Riordan, director of guidance at Cherry Hill's two public high schools.

"There's a part of me that says it's unfair, but those are the decisions that children today have to make," he said.

Kingsway Regional High School in Woolwich Township requires juniors to complete a new questionnaire that asks, among other things, their philosophy of life, what they want employers to know about them, and the best advice they've ever received. Due date: June 1.

The questions are meant to get students thinking about their futures, jump-start their college essays, and help guidance staff with letters of recommendation, counselor Nancy McCloskey said.

"Everything has been moved up," she said. "We used to tell them they could work on their applications at Thanksgiving. Now if you do that, you're way too late."

It's a vicious circle, admissions officials say. Schools reject more applicants, so students hedge their bets and apply to more places. That takes time, which means they start the process earlier.

The glut of applications leads selective schools to reject even more students, said David Hawkins, director of public policy for the National Association for College Admission Counseling. The crunch is expected to continue through the decade.

The University of Pennsylvania reported an 11 percent increase in applications for September's freshman class over last year, but its acceptance rate was 14 percent, down three percentage points from 2006. Other Ivy League schools accept only 8 or 9 percent.

"There's a cascade effect, so colleges down the line are getting more selective as well," Hawkins said.

The average national acceptance rate remains steady at 70 percent, counselors remind students. But nothing induces panic in an ambitious junior like seeing brainy seniors rejected by the schools they had their hearts set on.

"You look at them and think, 'My grades aren't that good,' " said Irwin, 17, from Penncrest.

Parents are sympathetic even as they press their children to study, apply, decide.

"I feel so sorry for him," said Adam Hamburg's mother, Liz, an artist who attended Carnegie Mellon University. "It's much more than I ever did."

"I hate that colleges will ask you what program you want to go into. How do you know when you're only 17, 18 years old? I just don't think that's fair," she said.

Greg Pastino, an actuary whose daughter Anastasia gets straight A's at Cherry Hill High School West, remembers that he and 15 other members of his high school graduating class got into Penn.

"That would never happen today. The pressure on these kids is unbelievable," he said.

Linda Griffiths, guidance coordinator for the Pennsbury School District, urges juniors to make a plan so they don't get overwhelmed. And they can take solace knowing that if they get the hard stuff done now, senior year will be less hectic.

"Teenagers are resilient," she said. "From time to time they do get that look in their eye like, 'Oh, my gosh.' Then they bounce back."

Contact staff writer Kathy Boccella at 610-313-8123 or kboccella@phillynews.com.