I've just tested your code:
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
r = dt << Get Rows Where( :sex == "M" );
Show( r );
and I get the same results in JMP 14 and JMP 15.
Here's the log from JMP 14:
/*:
//:*/
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
r = dt << Get Rows Where( :sex == "M" );
Show( r );
/*:
r = [6, 7, 8, 12, 13, 14, 15, 21, 22, 23, 24, 25, 26, 27, 30, 31, 32, 33, 34, 37, 39, 40];
and from JMP 15:
/*:
//:*/
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
r = dt << Get Rows Where( :sex == "M" );
Show( r );
/*:
r = [6, 7, 8, 12, 13, 14, 15, 21, 22, 23, 24, 25, 26, 27, 30, 31, 32, 33, 34, 37, 39, 40];
Can you give us some detail about what is failing for you? Is there an error message?
-Jeff