From what I understand, MAP testing is not a tool used to determine placement, as much as it is designed to measure academic growth throughout the school year.
That being said, if your child is testing at the 98th percentile at the beginning of the year, I would be concerned that the curriculum provided for the current grade level is not going to be challenging enough to teach him/her anything new and you might not see a whole lot of growth on the next MAP test.
Teachers can use the MAP results to differentiate instruction to meet each individual student's needs, however, you have to have a teacher that understands the MAP results and is on board with challenging the kids who have the higher MAP scores.
If your child is not at a school that has a G&T program, I would ask for extra challenges in Reading and Math and if they cannot accommodate you then I would definitely continue to supplement at home with more challenging (more interesting to your child) work in those two areas.
I cannot recommend any direct method on figuring out what grade level work your child will need to feel challenged, but what I do for my own daughter is find things that she is interested in learning about (lately it has been the Moon) and we do research together and write papers. As far as Math goes, I have a pretty good idea of the concepts that she knows and what she is lacking in, so I purchase a Math workbook that offers growth without being too complicated. My DD loves algebra, so often times I just find or make up simple algebra equations and let her solve them. As far as Science goes, we purchased a book of scientific experiments and do them at home and record our findings, as well as some basic chemistry and biology research on things that she is interested in.
Because my DD had lower scores on her last Reading MAP test, I purchased a 5th grade vocabulary workbook and we are going to start working on strengthening her vocabulary there. The 4th grade book looked too easy, so I opted for the 5th.
