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If I purchase 15 BarCode scanners and place in each class and connect all with Main Control PC by using USB HUB..
I think it is a wrong solution, simply because USB is very limited on the length of cable, it is about 5 to 10 meters maximum. I fear your classrooms will not be that close.
You need to gather informations about how far away are the classrooms and see with manufacturer what are the possibilities.
I would certainly choose a scanner with an ethernet link and serial emulation.
The PC having a driver that show a virtual RS232 per scanner.