To face a boss, you must first defeat all gang members from each turf at least once before entering a Boss' turf. Also, you must defeat the bosses in the order below to enter Reihou High School. Sawaguchi: Hanazo Second District (just before the second shopping arcade), and again at Marukaunsou Haisoko Higashiguchi (back gate of the warehouse after defeating Nishimura).

Kamijou and Yamamoto: Below Shintakaragawa (below the bridge after the 2nd town)
& again at the sauna (after defeating Nishimura, Mochizuki, and Taira).

Nishimura: Marukaunsou Haisouko (inside the warehouse)

Mochizuki: Marukaunsou Haibiru Shoumenguchi (outside the tunnel)

Taira: Midorimachi Third District (just before the third shopping arcade)

Kinoshita: Hanazo Park (after defeating Taira)

Kobayashi: Kobayashi Sangyo Kojou (inside the factory, after defeating Kinoshita)

Kunio or Riki (only when playing a solo game): Kobayashi Sangyo Kojou Uradeguchi (back of the factory, just before the last shopping arcade)

Himada: Yumemichou Second District (after the sauna)

Gouda: Outside Reihou High School

Onizuka: Reihou School Gym

Gotai: Reihou Classroom Third Floor

Ryouichi and Ryouji (Billy and Jimmy Lee: The Double Dragon Twins): Reihou
Classroom Top Floor

Yamada: Reihou Classroom Rooftop