You should apply restriction on the Parent’s Role on all the Doctype, and define User Permission based on Student. With this, when parent logs in, they will only be able to see data for their kid/Student.
I am able to set permission student level by going to " User Permission Manager". I have one more problem. This page gives me text box to type user name, doctype, and record(student in this case). But record textbox is listing down all the student. So one has to know which studnent are kids of particular parent. How can i make listing down only student who are their kids.