The grades that save slackers are disappearing from report cards at several Silicon Valley high schools.

Students from Mountain View to San Jose to Fremont are learning that if they don't work hard enough to earn a C-minus, they flunk.

A growing number of teachers have eliminated D's, betting it will boost students' achievement by heightening their fear of failing. Most high schools still allow students to graduate with D's, but many four-year colleges don't recognize such a low grade under admission requirements.

``Where else in the world does anyone accept `D work' but in public schools?'' says Pete Murchison, principal at Fremont's Irvington High School, which has done away with D's altogether.

Taking D's out of the equation is entirely up to each teacher, according to the state education code. A Mercury News survey found teachers at six of 23 Silicon Valley public schools recently stopped doling out D's.

Some of those teachers lower the C-minus cut-off to 67 percent. Others hold the line at 70 percent, making anything below that an F. In most cases, teachers have devised ways to help students who are heading toward failing.

The demise of the D makes it harder to pass a class, but educators say it's improving marks in their grade books. Still, some wonder whether the new grading scheme demands too much from students who aren't shooting for spots at Stanford or even Cal State-Stanislaus.

``I'd rather go to a junior college,'' said Alex Johnson, a junior at Mountain View High who is eyeing Foothill or De Anza community colleges. He says it's unfair that some teachers at his school are widening the range for an F. His dad isn't thrilled either.

``D's are the only thing keeping him from getting F's,'' Alex's dad, Doug Johnson, said. ``He's an incredibly bright kid, but he couldn't care less about school.''

That's precisely the problem, say teachers who don't want to pass students who scrawl their names and some answers on exams but still don't grasp much of the material.

``I got into this business to educate them, not to fail them, '' said math teacher Jake Schwartzberg, one of several Mountain View High teachers who has erased D's from the grading scale.

He said most of his students who would settle for a D -- if he offered it -- instead study a little harder.

``I intervene. I get in their faces. I prod, cajole, do whatever I can'' so they learn enough to at least earn a C-minus, said Schwartzberg, who's been teaching for 11 years.

``I bring them in at brunch and lunch and after school and weekends.''

The University of California and California State University systems don't accept D's on high school transcripts, forcing applicants to retake the required courses and earn at least a C-minus. So, ``it doesn't make a lot of sense'' to give that grade, said Santa Clara High School Principal Brad Syth.

Doing away with D's has motivated senior Atif Kamran, who got his academic act together at Irvington.

``My view of school before was an obstacle I had to go through every day so I could go home and play video games,'' said Atif, 17, who ``cruised through'' middle school with B's, C's and D's. The threat of flunking, though, prompted him to do just enough.

Midway through freshman year, Atif started failing Spanish. ``I worked harder,'' he said, and scored a C.

Now Atif said he cares about all of his courses. He pays more attention in class and talks to his teachers. The result is report cards full of A's and B's, even in his two college-level Advanced Placement classes.

High school principals and education experts give the No-D concept mixed reviews.

Stanford education Professor John Krumboltz, whose expertise includes grading policies, said the idea could run rampant until teachers demand perfection.

``If you're going to eliminate the D, you could say `Why not eliminate the C?' and make everyone work for an A or B,'' he said. ``The next step after that is make everyone work for an A.''

But his colleague Deborah Stipek, a Stanford education professor who focuses on student motivation, said ``it actually sounds pretty clever.''

In San Jose's East Side Union High School District, some teachers at Piedmont Hills High School don't offer D's. And teachers at Independence High are thinking about following suit.

But elsewhere in East Side, Evergreen Valley High Principal Tim McDonough is pulling the plug on the grading experiment. The school will revert to an A-through-F system in the fall because he's concerned about grading inequity.

Only 25 percent of Evergreen's teachers do not award D's. That means a student who flunks under one of those teachers -- and is forced to retake the course -- could have skated by with a passing grade from a teacher who did offer D's.

Irvington High School, which led the movement to drop D's more than a decade ago, has seen grades rise.

About 14 percent of Irvington's freshmen earn the equivalent of a C-average or lower; 9 percent achieve an even mix of A's and B's, or better.

By senior year, however, the percentage of students with a 2.0 GPA or lower slips to the single-digits. And students with a 3.5 GPA or higher doubles to 20 percent.

``What we try to do is change people's habits,'' said Murchison, the principal.

The message that ``substandard work is OK'' doesn't translate well into the real world, he said.

``I'm fixing my kitchen right now,'' Murchison said. ``I'm not going to pay a guy $5,000 for `D-work'.''