My Sister, My Science Report, by Margaret Bechard, 1990

Which is worse? Being assigned a science report on barn owls, or being assigned nerdy Phoenix Guber as a partner? Tess isn't sure, but possibly neither outrage trumps having a teenaged older sister. When Phoenix and Tess decide to ditch barn owls and instead study a teenager in her natural habitat, mixups follow thick and fast, and Tess's scientific observations show her that people are not always what they seem. Recommended.