For ACM and IEEE programming contests they used to be conducted "offline". Not sure how they're done these days, but participants would have a computer connecte

For ACM and IEEE programming contests they used to be conducted "offline". Not sure how they're done these days, but participants would have a computer connected to a LAN and shared resources for themselves and their team (one or more computers depending on the particular format). Put students in a room with a disconnected computer and an IDE or a computer connected only to a LAN. Submissions can be handled either with a local contest server or sneaker net to the one networked computer managed by the proctor. It's also possible to severely lock down the network connectivity of the computers so they cannot access the internet at large but only the contest servers during the competition if there is an online submission required.