This is very school-dependent. My class recently took the BSCE (basic science comprehensive exam) put out by the AAMC(? or however makes the STEP I) and most (the great majority of us?) got failing scores. The administration themselves admitted that this is normal as our school does not prepare us for the boards (they do not "teach to the boards") and the class does alright (mid 230s?), so diagnostic test performance can be very variable.