Your 2nd grade teacher's explanation is not exactly correct. RIT scores are continuously scaled across grades, using a version of Rasch scaling, which attempts to line items and performance up in order of difficulty. They are not deviation scores (as WISC or WJ scores are), which assume a bell curve, and compare performance to a standardization sample of some kind of peer (grade, in this example). While it is true that the RIT score does not mean exactly the same thing for a 2nd grader as for a 3rd grader, nor should they be interpreted as absolute grade levels, a 230 does not mean that a 2nd grader is as far from the 2nd grade mean as a 3rd grader is from the 3rd grade mean. That would be the case only if it were a deviation measure.
It is possible to find grade-based percentile scores to accompany RIT scores. Those would be different for each grade (i.e, the same RIT score would be at a different percentile in a different grade).
Grade equivalents mean only that the score obtained by this student is the same as the median score on this specific task obtained (or statistically predicted to be obtained) by the members of the standardization pool who were at this named grade level. They do not mean that a student has mastered the content expected of a student at that grade level, nor that they should most appropriately be placed at that grade level for instruction. Given the number of items administered in a MAP probe (about 25, give or take), there is no way that it can be a comprehensive assessment. It's a sampling of skills that we use to predict/estimate the actual range of skills mastered.
MAP is not actually designed to individualize teaching. It's really for at-risk screening, progress monitoring, and creating instructional groupings.
