What is the ruling on speaking and sitting with girls at the university or via WhatsApp for study purposes and others, knowing the difficulty of lowering one's gaze, and is that considered impermissible (haram)?

Speaking with a non-mahram (definition) woman without necessity is a door to fitna (temptation) and a means to corruption. Scholars have issued fatwas prohibiting it, so it is not permissible without a need. However, if there is a need, such as for testimony, trade, or conveying information, then it is permissible. One must be cautious of this matter, lower their gaze, and avoid suspicious situations to protect one's honor and to prevent people from belittling the bearers of the Quran.