Lehman High School students were warned Friday they should remain vigilant about AIDS, or risk joining 20,000 Bronx residents a group says are infected with the virus that causes AIDS.Representatives from Preventing HIV AIDS in the Next Generation (PHANG) reminded students to practice safe sex, keep their heads clear and make good choices. The group cautioned that while using condoms reduces the risk of transmission, there is no guarantee those who wear condoms won?t get the virus. One man who spoke on behalf of PHANG told students he used a condom that broke. He later tested positive for HIV.According to statistics gathered by PHANG, 31 percent of the 20,000 Bronx residents infected with HIV are teens, almost half of whom don't know they have HIV. The group says throughout New York City, teens under the age of 18 are eligible for free testing. The teens can be tested without their parents? consent.