I was very lucky enough not to see anything (CHOY!! I don't wish to see anything either).
I remembered 6-7years ago... I was around 13-14 years old.
I was with 1 of my best friend & several male school mates hanging outside. Feeling bored, (as it was december holidays, NO SCHOOL) 1 of the guy suggested we explore our school in dark. (Means illegally climbing in)
We agreed because our main purpose was actually to see "ghost" & look what was like in the dark. So we headed back home first, bring some of our stuffs like towel. Because we intended to stay inside one of the classroom to relax after exploring.
Met outside our school (all stay nearby only) & we climbed in by the side gate where there's no people, dark & easy to climb (No sharp thingy on top to prevent us from climbing over)
Summarize abit. We wanted to go to the 3rd storey. But all our staircase are locked with those metal shuttles. So we went to the back of the canteen where there's also 1 staircase, thou locked with metal shuttle, but we can climb the side to the staircase. (Only staircase without window from blocking us getting in). We tried to climb up but failed attempts.
So the guys went over to the canteen to bring 1 of the table over so we can climb up without any hassle.
Ok so they asked me to climb up first. (Ladies first) I climbed up & heard a very loud slipper sound. (Those dragging on the floor type) I thought we got spotted by those cleaners still in school

..
I was prepared to get caught seriously. (Thou it's like 12am++ already). But once the slipper walked past the staircase we were climbing at, the sound disappeared. (I was like "WTF".) But I kept quiet so as not to scare my friends. They reacted very calmly, so I think they didn't hear what I heard.
Brushing away the thoughts that it might not be real. So, we headed over to the 3rd storey. Went into 1 of the classroom. We got bored & started to play "Truth or Dare". We did funny stunts like kissing the white board, kissing the floor, smelling 1 of our friend's foot

.
Then we dared one of our male friend, to go over opposite of the classrooms & wave at us.
He agreed. Walked slowly towards the classrooms opposite of us. (Act until very man sia) Then he waved at us. Then shouted saying he's not scared at all!
Then I told my friends inside the classroom, I wonder what would he react if someone tapped his shoulder HAHA. But of cause nothing happened.
(Realized I'm being very long-winded) Cut short to the main point.
Boring of the games we played. We decided to really explore the school in darkness. 1 of the guy was tired so he said he's going to sleep in the classroom while we go ahead.
So will be left 2 girls 2 boys. (Including me)
Well, we wanted to head to the computer lab when 1 of the guy said, let's turn back. We still wanted to walk ahead & he repeated, "let's turn back. It's spooky."
We didn't ask more. We turned back, but not back to the classroom.
We walked to the other side, where the long stretch of classrooms were.
Suddenly I heard someone playing basketball. (WTH

.... so late still got ppl play basketball ah


) So i press my ears against the window of the classrooms. I still hear it very very clearly. But the sounds seems so unreal. (Can differentiate between real basketball sound & UNREAL basketball sound.)
I decided to open the door of the classroom to go further inside to the other side, to look down the window see if anyone is playing basketball.
When I wanted to go in the room, my friend said she's scared & didn't want to go ahead. 1 of the guy said he wanted to go too. Apparently he heard the sounds, but the other 2 didn't hear anything. So me & 1 guy went into the classroom & looked down. No one was there. The sound of the basketballs gone.
We headed out of the classroom. Feeling quite eerie , we still continued walking. Then we heard something like someone blowing their hair with a hairdryer at the room on top.(on the 4th floor) We didn't want to go up and see. We just stood on the 3rd floor & looked up. We found out where the sound came from. From 1 of the room at the 4th floor. We assumed it was air con's machine sound because no one will actually dry their hair in the middle of the night right

(especially in a school)
We didn't know what the room contains because 4th storey is for upper secondary student's classroom.
We quickly went back to the classroom & talked till morning. (Was very lucky we didn't see anything!)
Okay, 1-2years passed by (can't remember exactly when) I was in the upper sec. Classroom at 4th storey.
My male classmate disturbed me... & I chased him while he ran out of the classroom. I was tired after chasing & walking back to class, I saw some cleaners cleaning a room. I remembered it was the room we heard the sound from (As there's only 1 room located there on 4th storey. No classrooms beside it) I peep inside. No air con because it's like a store room for drawers, desks & chairs.
Not scary at all la.. But just wanna share my boring story