This is a true story that happened when I was around 11 years old. It is, so far, the only ghost experience I have ever had, but it was one of my scariest moments.
I was in middle school when this experience occured. We have two bathrooms for girls and boys. The downstairs bathrooms and the upstairs ones. Everyone knows about the upstairs girl's bathroom. The reason is, whenever you go in there, you always feel... Really weird. It feels cold, and basically, just gives you a feeling of distortion. Because I was in with the older students, all our classes were upstairs. So if we needed the toilet, we had to use the upstairs ones.
Anyway, one lesson, I needed to go really bad. I had no choice but to use the upstairs bathrooms. I walk in to the girl's bathroom, and immediately get that familliar sense of disorientation. I rush in to the cubicle, all the while feeling freezing cold (despite it being a lovely warm day). My heart was pounding, the feeling of panic was rising. I just wanted to get out of there quickly. I opened the cubicle door, and headed for the sinks. Even though I was terrified, I was still a freak about germs. I quickly washed my hands, and glanced at the mirror in front of me. My heart literally stopped. My eyes went wide. In the mirror's reflection, I kid you not, was a young girl right beside me. She was turned to the left, making a clapping gesture with her hands, eyes closed. I only saw her for about 2 seconds, before I shot out of there, screaming.
I assure you, I was the only one in there the entire time. The bathroom door is a very large one, you can certainly hear it if someone comes in, or leaves. When I first entered, the room was empty, and all the cubicle doors were swinged open. So I am positive, nobody was in there but me. Besides, when I left the cubicle, it was still empty. I only saw this girl in the reflection.
I never went to that bathroom again, and now I've moved up to Highschool. My friend has had a similliar experience in the same bathroom, but that's for another day.