Since this isn't the first time you've posted about taking photos for this little art school, maybe you're at the point where you need to talk to them and/or decide if you should just move on.
If you've been providing photos for free I'm not sure if they'd want to start paying you now or not. I'd talk with probably the person that you set this up with in the first place and discuss your concerns. You could take pictures for them providing your time & talent as a donation, or they could arrange payment for the service for a fee (out of their budget for operating expenses etc.).
Some of your concern seems to be about usage so you probably need to get clear how they'll use your photos. One of the situations you mentioned before was I think an instructor using your photos on her own page - that to me would seem to be inappropriate and unacceptable. It would seem that the usage would be for the school only to use on their website and social media page(s), and if an instructor wanted to use some of your photos you'd need to contract directly with that person and determine your pricing for that.
And you don't need to give them all the pictures you take. Pick the ones you want them to have/use; that could be a periodic donation and if they want more than that, you might need to contract and charge for more extensive usage if they want photos of all their openings, events etc. Talk to them about what you think seems appropriate as a donation on your part if you want to continue doing this.
This seems to have been a learning opportunity for you, so I'd take into consideration that they provided you with a way to learn and practice your skills. If you're at the point that you want it to be more than that you'll probably need to talk to them about it.