Tunnel Rat and 7 caves

Yes. There are not 5+1(end) caves in Hana Gakuen.

Basic 5+1

So, you ran around the whole map and found these caves:
1. near the entrance
2. at the entrance to the theater area
3. at the theatre
4. at the end of the river
5. at the shrine
and 7. on the river for a cutscene

A secret one

But to open the secret cave, you will have to do a little ritual with Mugen.
1. Take this particular stone
2. Carry the stone through the exit gate
3. Carry the stone through the first cave and put it a stone in a dead end.

Now you have access to the restricted area for any character. It's also easier to get an achievement "What's Done is Done"

Bug

There is a bug in the game right now. If you open 5 tunnels and skip the last cutscene by holding the Spacebar, the achievement will also be completed.

[ilares]

note if achievement doesn't unlock: don't skip the exit cutscene at the end of the mission (as you will open the last secret tunnel)