Is it allowed for me as a man to advise my classmate to wear hijab and modest clothes?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.

Originally, the relationship between a woman and any other man who is not her husband, nor is he a mahram must be restricted. If there is a necessity to communicate, this communication must take place under conditions: being polite and modest. It is not your business as a man to advise your female classmate to wear hijab. She might reject your advice. She may respond negatively to your advice. Thus, you might feel embarrassed.

Such communication may lead to haram relationship. People think that as long as we talk politely to each other, and we urge each other to do good deeds, the relation is allowed and acceptable. However, this could be an entrance to haram relationship that may take place later.

It would be much better to let her female friends advise her to wear the hijab as they can be closer to her, and they have much more time to talk to her. In addition, you can ask her professors who are old enough to talk to her. This may be more convincing.