you shouldn't use hub for

sharing internet unless each of you guys has unique ip (which is not common since you guys are not accessing the internet through a school resnet connection using dhcp). The collisions are due to the fact that incoming traffic and outgoing traffic don't know where to go.
What you need to do is purchase a router or use one of your guys' computer as a router server to rout traffics via routing software such as wingate.

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