By STEPHANIE STROM

Educators are giving YouTube — long dismissed as a storehouse of whimsical, time-wasting and occasionally distasteful videos — another look. As Google, YouTube’s parent company, fine-tunes a portal that lets schools limit students’ access to selected content, the video-sharing Web site is gaining popularity as a trove of free educational materials.

Schools across the country commonly block access to YouTube, shielding students from the irresistible distractions of, say, the cat in a T-shirt playing a piano, or worse. So in December, Google started YouTube for Schools, offering schools the ability to pluck only the videos they want, scrubbed of all comments and linked only to other related educational videos. The program gives schools the ability to allow access to the YouTube EDU educational library, and to specific videos within its own network — while blocking the general site.

That has enabled teachers to bring popular educational videos from YouTube into classrooms, like the famous TED talk on population growth by Hans Rosling, the Swedish data presentation expert, or a series of hugely popular short videos about each element of the periodic table that somehow turns a rote memorization exercise into gripping entertainment.

Slowly but surely, schools are taking down some of the barriers. “We’re really excited about it here,” said John Connolly, educational technology director for the Chicago Public Schools, which began allowing teachers to use YouTube for Schools last month. “We’re making content and tools available to our teachers to help them increase and enhance their teaching.”

Chicago is perhaps the largest school district to loosen its restrictions, but school technology administrators say it is just a matter of time until more barriers fall. At a time when financially ailing states are slashing public education budgets and there is mounting evidence of a widening achievement gap between rich and poor students, schools can ill afford to turn off a free source of credible, often premium, educational tools.

Robert Gulick, director of technology in the Washington Local Schools in Toledo, Ohio, said, “If we didn’t have a system for filtering it, we couldn’t partake, but we do now, and at a time of declining resources, it is a great way to find additional materials.”

Schools in the Toledo district previously allowed limited use of YouTube videos in class, but the process was cumbersome. Teachers logged onto a filtering system, and submitted a video for review by the technology department. If approved, the video could been viewed in the district’s “safe videos library.”

The new YouTube portal has made that process redundant, Dr. Gulick said. “Now students can safely explore on their own, and teachers are networking privately within grade level and building, sharing resources they’ve found,” he said.

In school districts where YouTube is blocked, teachers sometimes go to great lengths to show videos that they believe enhance their lessons.

“It can be a challenge,” said Jesse Spevack, assistant principal at the NYC iSchool in the TriBeCa neighborhood of Manhattan, which limits students’ ability to navigate and post on the Web from computers in classrooms and labs. “I’ve tried opening the window and loading the video on a laptop, or bringing a video in on my phone — or just asking the kids in my class, because there’s always some proxy hack site that a student will know how to use.”

When those techniques fail, he puts links to lessons on Khan Academy, TED talk videos and HipHughesHistory, a set of history-related videos created by a history teacher in Buffalo, on the class Web site, and asks students to view them outside of school.

Mr. Spevack said he understands New York City’s policy. “There is a lot of stuff on YouTube I wouldn’t be comfortable with my students seeing,” he said, “so I think trying to set up a way to differentiate content that is useful to schools and teachers from everything else is an awesome idea.”

Teachers have proved to be Google’s best emissaries for the filtering system, said Angela Lin, head of YouTube EDU. “The challenge now is getting these enthusiastic individual teachers to work with administrators and I.T. staff to make this a reality.”

Google has begun to create and solicit new channels in the hope of increasing its appeal. For instance, TED, a nonprofit group that works to spread the ideas of thought leaders from around the world, on Monday will start a channel that will eventually have hundreds of videos as part of a new educational initiative.

Brady Haran, the producer of the Deep Sky astronomy videos and Numberphile math videos, will develop two of the channels. “I don’t really think of them as lessons or teaching,” he said of his videos. “It’s far more useful to show something that’s tangential to the lesson, but supports it.”

Mr. Connolly, in Chicago, said the district had banned YouTube originally to ensure compliance with the federal Children’s Internet Protection Law, and to avoid jeopardizing federal financing for technology.

Over the last two and a half years, though, school administrators, teachers and students worked to convince the district of the merits of some YouTube content. That led to the creation of a committee to consider how the site might be filtered and, eventually, to a pilot program that was used in 15 schools.

“It has been a tremendous resource,” said Allan Fluharty, a teacher at Prosser Career Academy on Chicago’s West Side, which participated in the pilot.

Mr. Fluharty is teaching the concept of retrograde motion in astronomy. “You can describe it — a planet moving in one way and then all of a sudden moving the other way — and kids will be fine with that,” he said. “But if you can show them a video of it happening, then they will really understand what retrograde motion is in a much more meaningful way.”

His video playlist includes chemistry and quantum mechanics features, as well as videos on the history of astronomic theory.

Now that teachers in Chicago have access to YouTube, Mr. Connolly said the district would consider giving students access, something Mr. Fluharty favors. “It’s kind of like banning books, which we try to avoid,” he said.

More teachers are creating YouTube channels and playlists that are used as resources by other teachers and by Google itself. “Last summer,” Ms. Lin said, “we had a YouTube Teachers Studio, where teachers came from all over to train on how to use YouTube in the classroom as well as how to create video themselves. What we quickly realized was that some of them were way further ahead than we had thought and had already come up with some really innovative uses.”

Karen Mensing, who teaches gifted second graders in the Paradise Valley Unified School District in Scottsdale, Ariz., has her own YouTube channel that includes videos like “Confusing Words & Phrases” and “Using Technology in the Classroom.”

She makes videos of herself teaching so students who are absent can keep up, and of classroom activities so parents can see what’s going on.

“For Chinese New Year, I showed my homeroom class a short YouTube video of a New Year’s parade in China,” Ms. Mensing said. “That caught their attention so much more than seeing a picture in a textbook would have.”