When I was 13, I came out as gay to my only friend in school back then via message, hating myself for it, and the next day a friend of hers took her phone and read our chat history telling half of the Year I'm into boys. Sucked. Reactions were not too bad tho, could've been much worse.
This is why I made sure to specifically wait until school was over for me before telling my best friend I was gay. Those things always spread like wildfire in that environment. Plus, I guess it helped that I wasn't really sure of it until the age of 16, when I was definitely more mature than at 13.
