How can this student keep away from other students who are a bad influence?

First of all, it is essential to differentiate between mere supposition and imagination, and what it is feared may actually happen. Not everyone who makes friends with someone who is younger than him has bad intentions, unless he is known for doing bad things or he is not someone who is known for being righteous, in which case it is essential to be careful and take precautions.

In that case, what may be done is the following:

1. Alert his family and urge them to keep an eye their son and watch his behaviour, whilst trying to get to know his friends better.
2. Explain to him in a convincing manner the danger of making friends with such people, and the necessity of keeping away from them, whilst informing him of the intentions that some of them may have in making friends with him.
3. Explain to the students as a whole the rulings having to do with the ‘awrah and the obligation to cover it and guard it from people in general, and the obligation to avert the gaze, and to beware of mixing with bad friends.
4. Address those young men directly and advise them, and explain to them the warnings that the scholars gave against making friends with beardless youths, the danger of looking at them, and that the opinion of the scholars is that it is unlawful to look with desire at the face of a good-looking beardless young man; in fact some of the scholars stated that it is unlawful to look at him at all.

An-Nawawi said: The evidence for that is that is some ways he is like a woman, because he is desired as a woman is desired, and his appearance is beautiful like that of a woman. In fact, many of them may even be better looking than many women; indeed the prohibition may be more applicable to them for another reason, which is that in their case the ways of evil are more available than in the case of women.

End quote. Sharh Saheeh Muslim (4/31).

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The one who looks repeatedly at a beardless young man and the like, or persists in looking, and says, I am not looking with desire, is not to be believed, because if he has no reason for which he needs to look at him, then persisting in looking at him can only be interpreted as the result of the pleasure he finds in that. As for an accidental glance, it is pardoned if he then averts his gaze.

End quote. Majmoo‘ al-Fataawa (15/419).

1. If nothing of the sort is happening, but you still fear for him, then advise his family to move him to another circle, so as to keep him away from those mischief-makers.

And Allah knows best.