Hi I did this but only for a few weeks. I had thE site staff deep clean an unused disabled loo as it had a sink and a seat and was right by my area of school. I locked myself in at lunchtime and expressed every day, taking about 30 minutes. I took in a big Tupperware which I filled with water and Milton every day and stored the pump parts in there, then put the milk in our team room fridge nearby and took it home at the end of the day. I unembarrasedly explained to my team what the milk was so no one drank it, and to educate them a bit, and explained I would not be available between 12.50 and 1.10 each day. I was also let off lunch duty. It is a pain but it can be done!
I did too. I informed work I would still be feeding when I emailed them about returning to work and they were very supportive. The site team cleared out an underused walk in cupboard and fitted blinds and a lock for me. Head teacher arranged cover for my afternoon tutor period (20mins) and made it clear that I could continue for as long as necessary.
As it was, I found I only needed to do it for a month or so. After that my supply seemed to do magical things. I still feed ds (20mo) on demand during weekends and holidays but just morning and night in school days.
My advice is to tell school early and try to solve any problems (eg by suggesting times/locations) yourself.