一般什么时候是机票最便宜的时期？

There is no specific period that the airline ticket is the cheapest.
But if you check ticket price everyday, everywhere, you may be able to find some cheap tickets.
Also, the special fare for students is not always the cheapest, so don't depend on it.

只能天天查查看了。国内的代理有些都不是很靠谱。