Watertown-based Enanta Pharmaceutical’s Michael Tchen shows a group of WHS biology students around the company’s labs. About two-dozen Watertown AP biology students took part in what is becoming a tradition for Watertown High School when they got a look inside a real biotech lab that is working on cures and vaccinations for diseases, including HIV and hepatitis. The group of WHS students took the field trip to Enanta Pharmaceuticals Inc. on Arsenal Street earlier this month for what is known at the company as a Lunch & Learn. The group of mostly juniors and seniors got to see the different phases of developing a drug, and talk to people who working on the cures. Last March, the WHS AP chemistry classes took the tour of Enanta.