Is it permissible for female students to attend a private remedial lesson (three girls and a man) in a closed room, knowing that their faces are uncovered, and is that permissible if the lesson is merely for review and not for in-depth understanding?

The unlawful privacy (khalwa) is negated by the presence of more than one woman. The disagreement is regarding the unveiling of the face, and the fatwa (ruling) adopted by us is the obligation to cover it. If you believe it is obligatory to cover the face, then it is not permissible for you to attend these lessons. If you believe it is permissible to uncover it, then there is no harm upon you, provided you are safe from temptation (fitna). The more cautious approach is to refrain from these lessons as long as the person conducting them is a man. It is advisable to seek a woman to teach, if there is a need.