There isn't a single, readily available number for the high school graduation rate for all of St. Louis City. Graduation rates are often reported at the school district level, and St. Louis has a complex system with several districts (including the St. Louis Public Schools, as well as surrounding districts whose boundaries overlap with the city limits). Therefore, you would need to specify which school district you are interested in to get a precise answer.
To find the information, you would need to consult the data from the Missouri Department of Elementary and Secondary Education (DESE) website, which usually publishes graduation rate data broken down by district.