Nick, try this...

switch Score > 0 and Score <= 100 do
case Score >= 90 and Score <= 100:
"EXCELLENT"
case Score >= 75 and Score < 90:
"SATISFACTORY"
case Score >= 60 and Score < 85:
"NEEDS IMPROVEMENT"
case Score >= 45 and Score < 60:
"UNSATISFACTORY"
default:
"NOT SPECIFIED"
end

It looks like you have to specify the complete range you are testing for in the "switch" part and then test each segment in the "case" part. You left a 5 point gap between case statements so I adjusted for that. Also, you have to include the "Score" field in your case statements.

Sean