No, certifications are not grade-level specific, but most teachers make personal decisions regarding to what grades they want to work with. If she were to be successful with middle-schoolers (whereas she obviously failed with high-schoolers) wouldn't she feel that she has "found her niche"? Why would she want to apply for a job doing something at which she failed (teaching high-shool seniors) instead of something at which she succeeded (teaching middle-schoolers)?