He wants to get in touch with a Christian girl and marry her if she becomes Muslim

Firstly: 

We praise Allaah for having blessed you with faith and insight, which made you realize the seriousness of the matter from the outset and take this decisive step of changing universities. We ask Allaah to increase you in faith, guidance and piety. 

What happened to you is one of the bad effects of mixing in schools, workplaces and so on. Hence Islam in its wisdom forbids mixing. 

Beware of following in the footsteps of the shaytaan, for he will not try to tempt you openly, especially since he has seen your firm stance, rather he will come to you in a way that appears to be good, which is calling this girl to Islam and marrying her as Allaah has permitted. 

Allaah says (interpretation of the meaning): 

“O you who believe! Follow not the footsteps of Shaytaan (Satan). And whosoever follows the footsteps of Shaytaan (Satan), then, verily, he commands Al‑Fahsha’ [i.e. to commit indecency (illegal sexual intercourse)], and Al‑Munkar [disbelief and polytheism (i.e. to do evil and wicked deeds; and to speak or to do what is forbidden in Islam)]. And had it not been for the Grace of Allaah and His Mercy on you, not one of you would ever have been pure from sins. But Allaah purifies (guides to Islam) whom He wills, and Allaah is All‑Hearer, All‑Knower”

[al-Noor 24:21]

Beware of speaking to her on the phone, for fitnah (temptation) may result from listening to a person’s words as much as from looking, and being in touch may make you want to see her, and thus a person will get used to what he fled from, and worse. 

If the issue is simply to tell her that you want to marry her if she becomes Muslim, then this can be conveyed to her by a woman, but the matter will not stop there if you get in touch with her, rather she has to get to know you and get to know Islam, and find out how interested she is in forming a connection with you and so on, one thing after another, and these are the steps that come one after another. Then when love is established in the heart, it can only be expelled with difficulty, and that has caused some people to leave Islam, Allaah forbid. 

In some societies it is rare for a woman to convert from Christianity to Islam, then you will end up committing many evils in order to achieve what you imagine is a good end. 

In conclusion, we do not advise you to get in touch with her. It is better for you to forget about her, unless you can find trustworthy women who can call her to Islam and encourage her to marry you, without you having anything to do with it at all. 

We ask Allaah to protect us and you from all evils, trials and temptations. 

And Allaah knows best.