I could not find a serious answer to your question, so I came up with one which is mostly a jest.  So take it as one, please.  And no, I have not seen that in my city.

Maybe the reason is that it is the only way that they could sit down with their mobility severely curtail, so they had no choice but really study hard for the gaokao.  Maybe many of those still think baba and mama can still buy a better grade for them.

China, Japan and Korea really believe in these IV drips. If you're sick, even if it's just a cold, they will get an IV drip. Or, if they're really tired or generally feeling unwell, again, IV drip is a simple solution. The medicine will supposedly make them feel better. It is a bit extreme, and chances are that it'll be little more than a placebo affect, but look at the extremes people take to do well on this test. Considering that, it's not the most extreme thing ever done for the gaokao.

Yes, it's so common because of the fierce competition. They don't want to  fall behind  when they are sick. 

 I remember when I was in high school,   I often see students walking around with IV bottles in their own hands. They get up at 5 am, go to bed at 1 or 2 am.  Tons of homework need to be done everyday, they don't have time to think, just study study study like a robot. That's why the classroom are always quiet even during the break,they are exhausted! 

That's why they are always sick. That's why they can't afford to fall behind because there will be no time for you to catch up with others.

Sounds like a dumb solution to something that doesn't even seem that important.