Is it just me, or does Red High seem to employ Rubberband difficulty?

Red Leaf High has so many variables. As Untradeable pointed out, it scales based on your character's level. In my opinion, the low level scaling is ridiculously easy and fast, but once you start clearing 4th job and beyond, it only gets much harder.

There's also the randomization in what stages you get; two of them which force you to stay in the map for a set amount of time (60 secs with nurses, 120 secs with assassins). I typically clear the mob stages (bellflowers, undead) faster than "mini boss" stages (Kyrin, Dark Lord, Boss Kumi, etc). Then, of course, how fast you click Boss Kumi to put it your friendship points, and entering portals to advance.

Yep.

Edit: Oh, and let's not forget the crush stage (Mar, Laya, Shumi). One of the 4 options you choose lets you advance, so it might take time to get the "correct" option. :l