Is it impermissible to designate a WhatsApp number to communicate with 170 male and female students (boys and girls) for the purpose of answering their questions, sending motivational messages, and summaries, while maintaining boundaries so that no one sees the other?

If the work is limited to publishing beneficial messages and answering questions regarding studies and similar matters, there is no harm in it, and it does not fall under the forbidden mixing (ikhtilat) in Sharia. Rather, it is among the beneficial deeds for which its doer will be rewarded if their intention is sound and their aim is good.