Consider Dawn Ostroff the sorority mom.As topper of the CW, Ostroff has reason to feel proud of how the net has opened the 2008-09 TV season. So far, “90210″ is a Nielsen hit, and even the critical response wasn’t bad (low expectations probably didn’t hurt). Second-year series “Gossip Girl” remains buzzworthy, and newcomer “Privileged” came out of the gate in healthy shape. With the CW catering to an 18-34 crowd, what helps sell the skeins is the believability of the under-21 thesps that populate the shows: Shenae Grimes and Tristan Wilds of “90210,” Lucy Kate Hale and Ashley Newbrough of “Privileged” and Taylor Momsen of “Gossip Girl” (as well as Blake Lively and Ed Westwick, both of whom recently turned 21) have all resonated with teenage viewers. “It’s a great opportunity for these actors to prove themselves,” says Ostroff. “Typically, teen roles can be juvenile, but the characters these actors have picked are more realistic or heightened in a sophisticated way.” Ostroff, who credits her casting team in matching each actor to the appropriate character, says she equates the cast of “Gossip Girl” to that of a certain megahit NBC series. “The chemistry on ‘Gossip Girl’ is similar to ‘Friends,’” she says. “The first year of ‘Friends,’ the cast really took off and was photographed everywhere. That’s what happened on ‘Gossip Girl.’ It’s a one-in-a-million thing, like catching lightning in a bottle, and I think ’90210′ can do that as well.” “Gossip Girl” and “90210″ are hardly the first CW series to have filled plenty of teen actor slots. In fact, it’s something of a tradition for the network. “One Tree Hill” and “Everybody Hates Chris” are now in their sixth and fourth season, respectively. Ostroff says, “We’re lucky to have different shows that tap into a fantastic talent pool.” Recent breakthrough: “Gossip Girl,” “90210″ and “Privileged” each provide opportunities for young talent (and a few youthful twentysomethings) to act their age. What’s next: “Reaper” is a midseason entry for the CW. The net also just committed a put pilot for “The Graysons,” a look at Batman’s sidekick, Robin, before he became a superhero.