Shouldn't this be enough to solve the problem?
$ create table temp as select * from students;
$ insert into students_new (col1, col2, ..., mark_1, mark_2, mark_3)
select (col1, col2, ..., cast(mark_1 as varchar), cast(mark_2 as varchar), cast(mark_3 as varchar)) from temp;
I think that's what David wants to do.