Not!girl in school (personal story)

before i start this i want to say that i have never experienced something like a hallucination or similar thing, but im not dismissing that explanation to this story.

this happened when i was either in 8th or 7th grade, so 3 or 4 years ago. my (at the time) friend and i used to go the bathroom on breaks together. so on one of the breaks she asked me to go with her. we were few steps away from it as i saw a young girl with braided pigtails run up to the door, quickly open the bathroom door, walk in and close it. i looked at my friend and said "okay lets wait until she gets out" and my friend looked at me with questioning look " the hell are u talking about?" as she opened the door and we saw that no one was there. my friend laughed and told me i was crazy. our school bathrooms were really small so we would see at least her legs in one of the cabins (there were only 2 cabins and both of them unlocked). I moved from that school a year after, so did my friend, so i havent seen that "girl" since. 

i dont know what it was, and nor do i claim it to be absolutely paranormal or smthn. just it sits with me til this day and tought id share it.