As provocatively staged “reality TV” series explode on the commercial networks, PBS is expanding its own slate of what it calls “observational documentaries” in the network’s reinvention under President Pat Mitchell. In April , American High, a fast-paced docusoap series dropped by Fox last summer, will lead off a new weekly PBS strand targeted to teens and young adults. Then, in the fall, Senior Year, a 13-part series that PBS execs promoted to television critics during the January press tour, will take over the same Wednesday 10 p.m. timeslot. This summer P.O.V. also will present Fred Wiseman’s 1968 film High School, a national broadcast debut that was also promoted at the press tour. With American High and Senior Year, PBS aims to create a new strand, “so the audience can find it and know it’s going to be there,” and keep tuning in, said John Wilson, senior programming v.p. PBS has scheduled repeats of American High through the summer, and is considering a second run of Senior Year after its debut.