I'd ask the school what they use. I can understand the concern about whether being hungry and tired affects scores. My daughter was up all night before one test and had a high fever for another!
Longer tests like the WISC are considered the best and give you a fuller picture of strengths and weaknesses, but other tests (like the KBIT and RIAS) are shorter and may be less expensive. You could always do a shorter test and then a longer one later if needed. My DD took the RIAS and did so well that we did a full WISC-IV later. Get a full test if you can, but a shorter test may be less expensive if that is a concern. Some people do get big differences in scores (I've seen complaints about group tests in particular), but my daughter's scores were very similar on the KBIT-2, RIAS, and WISC-IV.
