Typed answers in a university's online exams

Your university runs its written exams online. Students sit in exam halls and type their answers into a web page on university laptops. Up to 3,000 students sit exams at the same time, and an exam lasts 2 hours. A student's typed answers for one exam come to at most 50 KB.

Last term the exam server crashed 25 minutes before the end of a large exam and was down for 15 minutes. About 900 students lost everything they had typed since they last pressed Save.

Hall Wi-Fi also drops now and then, for a few minutes at a time. A laptop can freeze or run out of battery, and the student then moves to a spare one. When time is up, the exam page locks the answer boxes by itself. Answers typed before the lock count if they reach the server within 30 minutes of the end. No single failure should cost a student more than 30 seconds of typing.

How do a student's answers get from the page to the server, and what happens when the server, the Wi-Fi or a laptop fails?