You can get the table variable's value from the string with
x = Current Data Table() << Get Table Variable( "teacher" );
And you can get a column's value with
x = column("height")[ 1 ];
Expressions can be used for more complicated meta-programming (JSL that manipulates JSL), but since you're starting with a string it may be better to use Parse() and Eval(). For example, the following takes a function name string and treats it as a function name.
s = "sqrt"; x = Eval( Parse( s || "(2)" ));