You might need an intermediate table to store this relationship, as it's possible that a student has more than one parent/gaurdian (a mother and a father) and it's possible that a parent/gaurdian has more than on student.
student------- idgaurdian-------- idstudent_gaurdian---------------- student_id gaurdian_id
The table student_gaurdian can store all the relations between students and gaurdians, and allows many-to-many relations.
RE: your new question
Having two tables with keys to each other looks a little weird to me, but if you really are having performance problems and this relieves them, then it's fine.
