sort a varchar field numerically in mysql
Two possibilities:
Either pad your values with zeros from the left and sort based on that:

SELECT _tid, _name FROM teacher ORDER BY lpad(_tid, 3, '0');
-- choose an appropriate number instead of 3


or cast your values to a number (similar to Aaron W's solution, apart from here I cast explicitly, and that is a clearer solution):

SELECT _tid,_name FROM teacher ORDER BY cast(_tid as decimal);


Be careful if you have non-number strings in your field:

SELECT cast('something' as decimal);
0
SELECT 'something' + 0;
0