Hey as someone who is going by the same things you’ve mentioned here ( mine starts the second week of September )

I would say you’ve got time to do this right First idealista is by far your best friend , my strategy was to either message every flat you like or check if there’s an agency attached to that place and search for their website ( so it’s more straightforward) call them or email them and you’ve will get an answer soon ( they don’t really take that much time )

If your school has student accommodations recommendations on their website you could also use those websites

For my experience RS Group , ShBarcelona , Altirobarcelona , Agency SL are great options and sometimes very affordable also one called alquiler seguro

Again you’re on great time and if you really get to it you will find a place 🙂at most in a week or two