What good a student can do to one's country?

'Become an educated, social aware, passionate citizen for Pakistan.'
Yes that sounds traditional but now after being in professional life for 5 years I truly know what that means which you cannot realize being s student so you also need to trust your elders :)
I have been so much with the welfare organizations and I loved doing that. But I paid for that in terms of loosing my concentration on studies and diverting interests. If you do good in studies you get good scholarships and eventually good jobs in the country and overseas, you can also become a good entrepreneur. And at that time you can do much more than you EVER expected. You have money, you have contacts, you have time, your plans are more mature. And I really feel that in current times no one has the right to ask you what you have done for your country and even if someone does don't bother answering. Just ignore all the negative energies. Break your side and rear view mirrors and drive straight towards your goal.

He should adopt the discipline and rules to prove himself or herself a good educated person of the society.

spread education should be the biggest return to the country according to me ... !

Students can help in causes to help needy, as they have more energy and strength to contribute as compared to others. They can educate those who can't read about traffic rules etc. There are many opportunities for youth to volunteer and I have seen that such activities are now really appreciated by schools and universities.