WHEN students showed up at school this spring with pornographic pictures from an electronic bulletin board, Caledonia High School cracked down.
The suburban Grand Rapids, Mich., school suspended four students from its computer network. School officials alerted parents and shut down all access to the Internet until this fall.
''We're trying to send a message to kids,'' says Mary Durkee, the district's Internet administrator who cleaned out the offensive material in the school's network. ''Don't try it, because it won't be tolerated.''
This is a problem that schools, parents, and legislators are just beginning to face. For all its educational potential, the Internet has a dark side. This global network of computers offers everything from sexually explicit on-line discussions to graphic, full-color smut. Such information is easy to find, simple to replicate, and hard to police.
Everyone from private software companies to a US senator is touting solutions. But the proposals are controversial, either because they are not technologically foolproof or because they run smack into free-speech rights. As the debate heats up, the first battleground is likely to be the classroom. Schools are poised to put millions of young people on-line, but many teachers and administrators have only a foggy notion of what is available on the Internet.
Only a tiny percentage of the Internet is obscene. ''The Internet is a little bit like Ivory soap,'' says Jay Friedland of SurfWatch Software in Los Altos, Calif. ''It's 99.7 percent pure. Unfortunately, 0.3 percent of that material is very sexually explicit.''
The explicit material goes far beyond the soft-core images of, say, Playboy magazine.
''What strikes me is the kind of pornography'' available, says Ron Charles, an English teacher at John Burroughs School, a private school serving Grades 7 through 12 in St. Louis.
Mr. Charles has never logged onto the more sexually explicit areas of the Internet, but his male students occasionally tell him about some of the material they find. ''It's bestiality, sadomasochism -- the kinds of things I didn't even know about when I was in high school,'' he says.
And it is very easy to find. The Internet offers no barriers to access. ''If anyone thinks that this material is hard for youngsters to come by, they don't know youngsters,'' writes Sen. James Exon (D) of Nebraska in a recent article for the Dallas Morning News.
''Does anyone really think that a parent can stand over their child's shoulder and monitor them all of their waking hours of every day?'' he asks.
Several companies are peddling products to filter out what comes over the Internet. SurfWatch, for instance, this week unveiled porn-blocking software.
''We originally thought the big audience would be parents,'' says Jay Friedland, marketing vice president for SurfWatch. But ''schools turned out to be the ideal candidates.''
Its $49 software screens patterns of key words in the addresses of Internet sites. If a site has the word ''porn'' in its address, for example, the software won't allow the user to reach it. The company claims its program blocks out 90 to 95 percent of sexually explicit sites on the Internet.
For an extra $5.95 a month, the company offers updates to screen out new objectionable sites. The company's software is available immediately for Macintosh computers. A version for IBM-compatible machines is due out in two months.
Senator Exon, meanwhile, has proposed a legislative solution. His ''communications decency'' amendment to a telecommunications reform bill in the Senate would toughen penalties on people who transmit sexually explicit material on-line.
It would double the maximum fine for such acts to $100,000 and quadruple the maximum jail sentence from six months to two years.
''He's not trying to be a censor,'' says the senator's press secretary, Russ Rader. ''He doesn't want to prevent people who are adults from having access to those kinds of things. He's just wanting to protect children.''
But civil libertarians worry that such measures would infringe the rights of free speech. And even the US Justice Department has weighed in against the amendment, warning in a letter that it ''would significantly thwart enforcement of existing laws regarding obscenity and child pornography, create several ways for distributors ... to avoid criminal liability, and threaten important First Amendment and privacy rights.''
Many schools are trying to address the issue themselves. They draw up acceptable-use policies and require both students and parents to sign them before the pupil can have Internet access.
Typically, these policies detail the ways the Internet should not be used, which could be anything from looking up basketball scores during class to downloading sexually explicit material.
Many Internet-connected teachers say the schools will have to do more. ''If we are not ourselves aware of what's out there and what [students] can become privy to, I think we're in a liability situation,'' says Susan Larson, fourth-grade teacher and computer-lab facilitator at Hemingway Elementary School in Ketchum, Idaho.