To what extent is the jurisprudential rule "Whatever leads to the forbidden is forbidden" sound, and how can one reconcile the prohibition of speaking with a non-mahram woman over the phone, based on this rule, with the permissibility of going to school despite the presence of forbidden mixing within it?

Means that are originally permissible become forbidden if they lead to something unlawful, as a way to prevent reaching the unlawful. This is the principle of "blocking the means" (sadd al-dhara'i'), which scholars have used as evidence, including the words of Allah Almighty: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge," and the words of the Prophet, peace and blessings be upon him: "Indeed, among the greatest major sins is for a man to curse his parents." It was said, "O Messenger of Allah, how can a man curse his parents?" He said: "He insults another man's father, so he insults his father; and he insults his mother, so he insults his mother." There is no contradiction in this principle with attending schools, as forbidden mixing can be avoided. However, if attending leads to forbidden mixing that cannot be avoided except by abandoning it, then abandoning attendance becomes obligatory.