MAP scores should be comparable across grades *if* the same version of the test is used. There are three versions, to my knowledge: K-2, 3-5, and 6+. Scores should be close to the same across versions but it's less reliable. aeh''s links give the information you need.
Playandlearn, I believe your teacher was misinformed; scores are not calculated based on the grade of the student. What you describe is how most other standardized academic testing works, but not MAP.
Keep in mind that the average student probably hasn't learned everything the class should have taught, and many high schoolers never succeed at higher math. Someone on this board told me years ago to compare to the 90-95th percentile MAP to find the true knowledge level of the student. This worked OK for my DD. If my kid had a 215 math score, I would try a beginning of 3rd grade placement test. (Most homeschooling curricula, like Singapore or Beast Academy, offer free printable placement tests.)
