1-1 - Shadow cat
it's on the cat house next to the level selector in the goblins hub.

1-2 - Welcome aboard!
It's on a tree behind the portal area at the end of the tutorial (there's a ticket sack nearby)

1-3 - Campus grounds
It's at the start of the tutorial on the notice board.

1-4 - Slime dispenser
It's on the notice board next to the bloodstains

1-5 - Join us
Right after the mimic area door turn left and go into a hidden area, the note will be on the big tree.

2-1 - Mana
It's on the notice board next to the torch

2-2 - Friend or foe
It's on the notice board next to the mimic

2-3 - Portals
It's on a notice board next to the portal

2-4 - No room for two
It's on the other notice board next to the portal

2-5 ???Entrance Tree title???
The note is on the wall at the end of the barrel room of the forlorn crypts.

3-1 - Disable eye
In the last room behind the laser eye this note is on the floor, impaled by a sword.

3-2 - Silence
It's on the pillar in the library section right before the 1st bookshelves start

3-3 - You always need a healer...
Beyond the locked door go outside where the moving fire turrets lay dormant and enter the 1st alcove where a book lies on the floor, go through the fake wall to find a note on the floor
Thx escorppion

3-4 - Skeletons
In the area where you kill a trapped skeleton with a lever you can find a note on the floor right in front of the bench and torch.
Thx Kojima

3-5 - Dark Tower Map #1
It's in the iron sewers in 1 of the cells behind the locked door

4-1 - Bolts
It's impaled on the wall in 1 of the cells in the dark area of Kobolds Alleyway

4-2 - Fire dragon
It's in the middle area on top of the pedestal, impaled by a sword.

4-3 - Engineer
In the devils throat area, after you get the lever, do not bring it to the bridge but instead jump down to the lowest area in the devils throat and find a fake wall in the alcove and use it there.
This will unlock a secret room at the bottom layer of the middle section, there you will find another lever, a sack of tickets and notes scattered on the ground.
Thx BunGirl for the tips

None?

None?

None?

4-4 - We need to fix this
This note is on the floor, impaled by a sword on a ridge, to find it you must go through the drawbridge and go forward and right, look for a fallen tree that bridges 2 places together, cross over it.

4-5 - Killer roots tree anatomy
On your way to the lever that lowers the draw bridge, right before the snowy stairs there are 2 broken stone pillars, the the right of them near 2 trees is a note impaled by a sword in the snow.
Thx escorpion

5-1 - Ice golem can be useful
Standing on the holy grenade and looking at the ice golem, turn to your right to the nearest tree, there next to it stands a sword impaling a note to the snow.

5-2 - This place is a death trap
Inside the inner hotsprings area there is room where it's a large square pool of frozen water with a magic book in the middle, on 1 side in that pool there is a small piece of land with a sword on it, the note is there, impaled by said sword.
You need to dip a slime into water to make it into an ice-slime and then poke that into the water to make an ice path.

5-3 - Hot Springs Map #1
Following the path from the starting point the note is on the notice board next to the path.

5-4 - Hot Springs Map #2
Following the middle path and crossing the river to your right, the note is on a notice board next to the platform with the telescope and mimic.

5-5 - Hot Springs Map #3
After the draw bridge go left up the river until the end and turn right, you'll find a notice board with a note on it.

6-1 - Hot Springs Map #4
Not far from the ice golem there is a platform with a gigantic brazier on it, next to it is a notice board with the note on it.

6-2 - Hot Springs Map #5
From the crossroads follow the path to the right and you'll find another notice board with a note on it.