Is it permissible for a male student to lead female students in prayer directly without a barrier, especially if they are not wearing the khimar and there is fear of temptation due to them, and what is the ruling on the behavior of the imam who accompanies them after the prayer?

Many scholars have permitted a man to lead non-mahram women in prayer, while some have disliked it.

Non-mahram students must not mix due to the risks that mixing entails.

If there is a mosque, there is no harm in women praying behind the Imam, provided there is no suspicion. However, it is better for them to pray in a special prayer area for women, and it is legislated for them to appoint one of their own to lead them in prayer.

As for covering women's faces, it is a matter of scholarly disagreement. The verifying scholars have made it obligatory when there is a fear of fitna (temptation/strife), and this is the correct view.