Is it permissible to teach in a free school funded by foreign organizations - including churches - given the teacher's knowledge that the number of students is less than what was specified, and is it lawful for him to take the salary?

There is no objection to receiving a salary donated by churches, and this does not necessitate prohibiting work at the school, because accepting gifts from disbelievers is originally permissible. As for the issue of the number of students being fewer than the number specified by the administration, receiving salaries is not forbidden, unless the school's governing body stipulated that the students must reach a specific number for salaries to be granted. In that case, salaries would not be permissible until that number is reached, because Muslims are bound by their conditions.