то, что вы хотите, это оператор switch:
=switch(
Fields!marks.Value<50,"Fail",
Fields!marks.Value>= 50 and Fields!marks.Value<65,"Accepted",
Fields!marks.Value>= 65 and Fields!marks.Value<75,"Good",
Fields!marks.Value>= 75 and Fields!marks.Value<85,"V Good",
Fields!marks.Value>= 85 and Fields!marks.Value<100,"Excellent",
,"")