While I'd be the first to say that a school must make allowances for religious observation...
Nowhere in the Muslim faith does it say WHEN this trip must be taken.
The teacher wants three weeks to make the trip. I don't know ANY teacher who can't get three weeks off over the summer, unless seniority demands they teach summer sessions.
So... My question would be, has this teacher had, at any time since 2008, three consecutive weeks off?
The school (or any employer) must make REASONABLE allowances for faith. Giving three weeks off during the school year, particularly when the same teacher/employee GETS a couple of MONTHS off is, IMHO, beyond reasonable.
CoH will be strictly enforced. Posts bashing religious as a whole will be deleted as off-topic.