I'm a bit surprised that there's no isolation switch for an extractor fan, but perhaps that's because you're in student accommodation and they worry that it will just be left turned off all the time.
Is this an extractor fan for a bathroom/shower room? This would explain concerns about mould.
But it's clearly an issue if it's preventing you from sleeping.
If this is accommodation arranged via your university's accommodation office, then I think you need to contact them and explain how this is affecting you. They may be able to raise this with the accommodation provider on your behalf.
Another option would be to raise a formal complaint with the accommodation provider regarding the situation, rather than just talking to the people on reception. You could ask your student union, or your university's wellbeing service, for advice and support on this.
Is this causing a problem for other people in your accommodation? Are the extractor fans in other rooms/flats set up the same way -- i.e. always on with no way to switch them off? If you can get other people to raise the same issue with the accommodation provider then that would give more weight to your complaint.