I agree with firealarminc’s guess above on how the Frankenstein system came to be, but it’s also not uncommon to have multiple vendors in a school like that. Public schools want whoever’s cheapest at the time so you end up with spaghetti systems like that occasionally. 
Some dealers also switch what they sell/install over time as products and codes change. I haven’t seen bells installed in a long time, so don’t expect them to be around much longer. It’s tough to get them from Wheelock these days for sprinkler bells. As for the EST with BG-12’s, well EST was bought out a few years ago so maybe a dealer wasn’t happy with the new agreements and jumped ship. There’s lots of reasons why different products get used over time!
People like to bash the proprietary systems, but it prevents stuff like this from happening and guarantee’s a life safety system that’s compatible and commissioned by someone who’s qualified.