Teenage sexuality, conformity and paranoia have been the ingredients of made-for-TV melodramas, movies and novels for decades. “Girl, Positive” (8 p.m., Lifetime) puts a new spin on the old “girl in trouble” genre and stars “90210” star Jennie Garth, to boot.

Garth plays Sarah Bennett, an HIV-positive substitute teacher who volunteers at an AIDS clinic run by Ariel (S. Epatha Merkerson). Sarah’s not exactly running to tell everyone about her status, especially not the handsome vice principal, who can’t hide his affections.

Sarah’s job gives her a window on the highly charged sexual atmosphere of a high school still reeling from the death of Jason (Eric Von Detten), a recent grad and all-around football hero. We soon learn that Jason was not as squeaky clean as his reputation. Before his car-accident death, he had been dabbling in heroin use and had contracted HIV. This revelation shatters his posthumous image and sends shivers of fear through the school’s sexually active population.

The film captures the emotional confusion of high school, a time when relationships can be casual and earth-shattering at the same time. The movie’s obvious message is that students’ rash decisions can have life-changing implications and that you can contract HIV from the “love of your life” or from a one-night stand.

“Positive” does a good job showing how the advent of text messaging and social-networking Web sites have turbocharged the velocity of teenage gossip. Ugly rumors that were once transmitted by whisper or furtive note passing now travel at the speed of light. The image of a quickly multiplying split screen not only demonstrates how the power of malicious hearsay has mutated since the innocent split-screen high jinks of “Bye, Bye Birdie,” it also provides a powerful visual metaphor for viral infection.

¢ “Fooled by Nature” (6:30 p.m., Animal Planet) asks viewers to distinguish fact from fiction. Every episode will showcase three fantastic examples of natural phenomenon, but only two will actually be true. You’ll just have to stay tuned to find out if you’ve been had.

Tonight’s other highlights

¢ Sigourney Weaver plays the mean taskmaster of a detention facility in the 2003 fantasy “Holes” (7 p.m., Disney).

¢ Pink-slipped on “Everybody Hates Chris” (7 p.m., CW).

¢ Kyle finds it hard to be normal on “Kyle XY” (7 p.m., Family).

¢ A tennis star tries to decide whether his score is 20-love or 40-love on “Age of Love” (8 p.m., NBC).

¢ The season ends for the “Ex-Wives’ Club” (8 p.m., ABC).

¢ A new season of “History Detectives” (8 p.m., PBS, check local listings) explores the stories that have grown up around family heirlooms and cherished possessions.

¢ Provenza’s card is found on a corpse on “The Closer” (8 p.m., TNT).

¢ “Simon Schama’s Power of Art” (9 p.m., PBS, check local listings) profiles Caravaggio.