my same school

Rather not. Better is "she is in the same school as me"

From Practical English Usage (Swan):
'We normally use the before same ... before a noun we use "the same as":

"You've got the same idea as me, (NOT ... my same idea)"
"Her hair's the same colour as her mother's" '

"she is in the same school as me" may be colloquially accepted, but should be avoided in formal writing.
"She goes to the same school as I do" and
"She is in the same school as I am"
are grammatically correct and are preferred, to my humble opinion.
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