- I would say that Mary lives ON Sucre Street, or she lives ON Mejia Street. She cannot live on both, UNLESS you want to say that she lives AT THE CORNER OF Sucre and Mejia Streets. (I believe that one can also say that Mary lives in Sucre Street, but I more commonly hear "on". That could depend on whether you are in England or in North America, but I'm not entirely certain).
- If you are not sure that Mary lives there, you would simple say so: "I think that Mary lives on Sucre Street". You can also add, "But I'm not sure".
- She is studying IN high school (or simply, she is IN high school), BUT she is studying AT San Pedro Pascual High School. Again, if you aren't sure if she is studying there, you would say so: "I think/believe/assume she is studying there".
- Michael is TAKING/GOING FOR a short walk UP/DOWN the street. If he weren't going for a walk, but was outside on the pavement, you could say, for example, that he was standing IN the street.
Hopefully, that all makes sense.