Calculated Column based on two column in the Same table

As a calculated column you could do this:

=SWITCH( TRUE(), 
Table[Subject Name] = "Math" && isblank( Table[Test Score] ), BLANK(),
Table[Subject Name] = "Math" && Table[Test Score] >= 90, "4.0",
Table[Subject Name] = "Math" && Table[Test Score] >=80 && Table[Test Score] <90,"3.5", 
Table[Subject Name] = "Math" && Table[Test Score] >=70 && Table[Test Score] <80, "3.0",
.
.
Table[Subject Name] = "Science" && isblank( Table[Test Score] ), BLANK(),
Table[Subject Name] = "Science" && Table[Test Score] >= 75, "4.0",
Table[Subject Name] = "Science" && Table[Test Score] >= 70 && Table[Test Score] < 75,"3.5", 
.
.
)

Obvioulsy I didn't fill out the whole thing, but you get the idea.  If you have a duplicative weighted schedule, you could "OR" the Subject Name checks in the SWITCH so as to not have to do a row for every subject.   Keep in mind the SWITCH will stop at the first TRUE statement.

Another approach would be to have nested SWITCH statements,  Outer switches on subject name, inner by score range.