why is this "es i i ysgol . . . ." whereas "I went to school" is "es i i'r ysgol". I still went to a specific school, even though it was English!
Because in this sentence you "have gone to AN english medium school", as opposed to "THE english meidum school.
To add:- if you're referring to a specific English medium school, which is quite common in some areas where there might be one English medium and one Welsh medium then you would use 'Es i i'r ysgol gyfrwng Saesneg'.