Mono chloro substituted products of naphthalene
Your chemistry teacher is partially correct by saying that naphthalene molecule is symmetrical about the axis passing through the $\ce{C-C}$ bond where the benzene rings are fused (going through the common carbons to both rings). Your chemistry teacher has forgot to mention that naphthalene molecule is also symmetrical about the axis perpendicular to the axis passing through the $\ce{C-C}$ bond where the benzene rings are fused (See the top structure of the diagram below). Thus, each quarter of the naphthalene molecule is equal and each quarter contains only two carbon atoms (disregarding common $\ce{C-C}$ bond). I named them as 1 and 2, because they are symmetrical. Therefore, only two different mono-substituted derivatives are possible from naphthalene. If substituent is chloro, then only 1-chloronaphthalene and 2-chloronaphthalene are possible, and so on.